implement webglcontextevent

[tylersouthwick]

fixes #10742

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This change is

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @larsbergstrom (or someone else) soon.

[highfive]

Heads up! This PR modifies the following files:

• @KiChjang: components/script/dom/document.rs, components/script/dom/webglcontextevent.rs
• @emilio: components/script/dom/webglcontextevent.rs

[Ms2ger]

Thanks!

@bors-servo r+

[bors-servo]

📌 Commit 0b5e6bd has been approved by Ms2ger

[jdm]

@bors-serso r-
./mach test-tidy doesn't pass yet according to TravisCI.

[emilio]

@bors-servo: r-

(Josh mispelled the bot name)

[Ms2ger]

So it seems:

./components/script/dom/document.rs:81: use statement is not in alphabetical order
    expected: dom::window::{ReflowReason, Window}
    found: dom::webglcontextevent::WebGLContextEvent
./components/script/dom/webglcontextevent.rs:44: extra space before :

[Ms2ger]

Thanks!

@bors-servo r+

[bors-servo]

📌 Commit cba0267 has been approved by Ms2ger

[bors-servo]

⌛ Testing commit cba0267 with merge 5920532...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel, mac-dev-unit, mac-rel-css, mac-rel-wpt