[Backport] gn: Remove unnecessary v8 defaults

[rakuco]

This is necessary for us to be able to change the value of
v8_use_external_startup_data in Crosswalk.

IMPORTANT: In M52 we only get rid of v8_use_external_startup_data, as
https://codereview.chromium.org/2024833002/ does not apply cleanly on
M52's V8. This patch must be rewritten for M53 to contain the entire
upstream change.

See also: crosswalk-project/v8-crosswalk#164

Original commit message:

Remove chromium defaults for v8_optimized_debug and
v8_use_external_startup_data.

This is not needed after v8 provides these defaults:
https://codereview.chromium.org/2025803003/
https://codereview.chromium.org/2024833002/

It also interferes if somebody tries to override the gn args
with a different value.

BUG=chromium:616034
TBR=alokp@chromium.org, brettw@chromium.org

Committed: https://crrev.com/0fffeb2adaa3c284b760922c1aecce1516b998ce
Review-Url: https://codereview.chromium.org/2058033002

[crosswalk-trybot]

Testing patch series with rakuco/chromium-crosswalk@0908bc7 as its head.

Bot	Status
Content Shell Android-x86	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Android-x86/builds/331)
Content Shell Linux	[SUCCESS 💚](https://build.crosswalk-project.org/try/builders/Content Shell Linux/builds/329)

[rakuco]

@darktears

@heke123: FYI.

@msisov @mrunalk: please see the part abou M52/M53 in the pull request message.

[pozdnyakov]

lgtm

[heke123]

@rakuco In BUILD.gn it imports this build_overrides/v8.gni, (not v8/BUILD.gn)
So I think we should set value to v8_use_external_startup_data here, and don't need to change in v8/BUILD.gn

In xwalk/build/common.gni, we removed the "defination" of v8_use_external_startup_data too, so "gn gen" currently complains:

  ERROR at //BUILD.gn:294:7: Undefined identifier
  if (v8_use_external_startup_data) {

[heke123]

Or I can add the v8_use_external_startup_data back inxwalk/build/common.gni. which one do you prefer? thanks.

[rakuco]

Ugh, good catch. GN support in M52's V8 is a pain in the ass to work with.

Setting v8_use_external_startup_data in src/xwalk is not enough, as the content shell builds would remain broken. For M52, I think the best course of action is to move the v8_use_external_startup_data from v8/BUILD.gn to v8/build_overrides/v8.gni and do the same thing in Chromium.