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Before:

<include src="...">
<if expr="chromeos">
    chromeOsOnlyCode();
</if>

After:

// <includes src="...">
// <if expr="chromeos">
    chromeOsOnlyCode();
// </if>

This is to unblock running clang-format on JavaScript and allow other
tools to run on more expected input inside of .js files (i.e. closure
compiler).

BUG=678778
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2624503002
Cr-Commit-Position: refs/heads/master@{#443069}