Various community-contributed externs updates.

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Reference document for HTML5 canvas, at http://www.w3.org/TR/2dcontext2/, shows that the 6th argument of arc ("anticlockwise") is optional.

Fixes issue 999

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Add Audio constructor.

Fixes issue 1013

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Add selectedOptions property extern

See
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#dom-select-selectedoptions

Fixes issue 1011
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=47411801
Move tryReduceReturn optimization.

This is really an alternate syntax substitution, and has nothing
to do with minimizing conditionals, so it probably makes more
sense in PeepholeSubstituteAlternateSyntax.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=47413309
Automated g4 rollback.

*** Reason for rollback ***

Audio constructor conflicts with w3c_audio externs

*** Original change description ***

Various community-contributed externs updates.

-----

Reference document for HTML5 canvas, at http://www.w3.org/TR/2dcontext2/, shows that the 6th argument of arc ("anticlockwise") is optional.

Fixes issue 999

-----

Add Audio constructor.

Fixes issue 1013

-----

Add selectedOptions property extern

See
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#dom-select-selectedoptions

Fixes issue 1011
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=47448890