Add GN build support for crosswalk-linux

[heke123]

As Chromium will deprecate GYP build system since M54, we need to enable
GN build support for Crosswalk as soon as possible.
Set xwalk as "extra_deps" to Chromium in xwalk/build/common.gni:
root_extra_deps = [ "//xwalk:xwalk_builder" ]

[crosswalk-trybot]

Testing patch series with heke123/chromium-crosswalk@d9f293d0faea58215f930ac00af500b059928bcb as its head.

Bot	Status
Content Shell Android-x86	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Android-x86/builds/327)
Content Shell Linux	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Linux/builds/325)

[heke123]

@rakuco

[rakuco]

The changes look fine to me, but they need to be committed differently: the enable_webvr part should be a separate commit backported from upstream (use git cherry-pick, edit the commit message to mention it is a backport like this one and make sure it does not contain the "Cr-Commit-Position" line) and then you can add xwalk to the exec_script whitelist in another one (and explain why it is needed).

[rakuco]

And there's nothing Linux-specific here, so I'd retitle the PR and not mention Linux in the commit where you change the exec_script whitelist.

[rakuco]

One last thing: don't you also need to port our chromium-crosswalk changes to GN as well? The following commits touch gyp files:

• c72323e ("[Extensions] Build glib message pump on Ozone by default")
• 29782db ("Add flag to disable hrtf in webaudio")
• 84686b5 ("[Windows] Support RealSense Cameras")
• 63e1f41 ("[Temp] Do not enable generate_multidex_config by default on Android.")
• 2b2cbc8 ("[Windows] Implementation of 'AudioDestinationNode.devicePosition' attribute")
• ad2a7f0 ("[Temp] Make |enable_webvr==0| work.")

Most of those are not related to Linux so you don't necessarily have to port all of them here, but I thought it'd be good to bring them up in case you were not considering them.

[heke123]

@rakuco Thanks very much for remind me this.
for this one:
2b2cbc8 ("[Windows] Implementation of 'AudioDestinationNode.devicePosition' attribute")
The title is for windows only, but seems the gyp modification has affects on all platforms, should I port it now?

And below 2 commits will be port into GN soon.
c72323e ("[Extensions] Build glib message pump on Ozone by default")
29782db ("Add flag to disable hrtf in webaudio")

[rakuco]

The title is for windows only, but seems the gyp modification has affects on all platforms, should I port it now?

Yes, I think you'll need to port this one too (it should be a matter of adding a few lines to some BUILD.gn files).

[rakuco]

63e1f41 ("[Temp] Do not enable generate_multidex_config by default on Android.")

There doesn't seem to be anything to do with this one: as far as I can see, each target needs to set enable_multidex = true for the multidex stuff to be enabled (i.e. it becomes opt-in instead of opt-out, which is what we want)

[rakuco]

FYI, I'm handling the Android-specific changes in #377.

[heke123]

@rakuco
2b2cbc8 ("[Windows] Implementation of 'AudioDestinationNode.devicePosition' attribute")
It contains the BUILD.gn. so we don't need do anything.

You intent to remove below 2 commits, I guess we don't need to port them either?
c72323e ("[Extensions] Build glib message pump on Ozone by default")
29782db ("Add flag to disable hrtf in webaudio")

[rakuco]

2b2cbc8 ("[Windows] Implementation of 'AudioDestinationNode.devicePosition' attribute")
It contains the BUILD.gn. so we don't need do anything.

Excellent!

You intent to remove below 2 commits, I guess we don't need to port them either?
c72323e ("[Extensions] Build glib message pump on Ozone by default")
29782db ("Add flag to disable hrtf in webaudio")

Yep, correct.

[crosswalk-trybot]

Testing patch series with heke123/chromium-crosswalk@ffc2b3214a333fda6e31f3a724bdd951ec265ba3 as its head.

Bot	Status
Content Shell Android-x86	[FAILED 💔](https://build.crosswalk-project.org/try/builders/Content Shell Android-x86/builds/332)
Content Shell Linux	[FAILED 💔](https://build.crosswalk-project.org/try/builders/Content Shell Linux/builds/330)

[crosswalk-trybot]

Testing patch series with heke123/chromium-crosswalk@bde5610d5025d1ec94dee063eba78d8a6012d5e5 as its head.

Bot	Status
Content Shell Android-x86	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Android-x86/builds/333)
Content Shell Linux	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Linux/builds/331)

[rakuco]

yes! victory at last 👍

[heke123]

Thanks so much for your help!