Add get/set pref extensions to webdriver server.

[jgraham]

This can be used by the test framework to ensure that the correct prefs are
set for a test without restarting the browser

[Ms2ger]

Review status: 0 of 4 files reviewed at latest revision, 4 unresolved discussions, some commit checks failed.

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^{components/webdriver_server/lib.rs, line 85 [r1] (raw file):}
Too long

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^{components/webdriver_server/lib.rs, line 158 [r1] (raw file):}
Nested try is very hard to read.

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^{components/webdriver_server/lib.rs, line 165 [r1] (raw file):}
(x, y)

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^{components/webdriver_server/lib.rs, line 634 [r1] (raw file):}
Better as

let prefs = parameters.prefs
                      .iter()
                      .map(|item| (item.clone(), get_pref(item).to_json()))
                      .collect::<BTreeMap<_>>();

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Comments from the review on Reviewable.io

[metajack]

Shouldn't there be a third Cargo.lock update or does webdriver_server not get used on Android?

Otherwise looks good.

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from the review on Reviewable.io

[jdm]

It's an optional cargo feature that doesn't get enabled on android right now, presumably.

[jgraham]

I don't know, I just trusted |mach cargo-update|

[metajack]

@bors-servo r+

[bors-servo]

📌 Commit e0a7b2b has been approved by metajack

[bors-servo]

⌛ Testing commit e0a7b2b with merge 6703e9e...

[bors-servo]

💔 Test failed - linux-rel

[jdm]

@bors-servo: retry

• #7437

[bors-servo]

⌛ Testing commit e0a7b2b with merge 67cf241...

[bors-servo]

☀️ Test successful - android, gonk, linux-dev, linux-rel, mac-dev-ref-unit, mac-rel-css, mac-rel-wpt