Fix grit's <if> not to strip trailing or preceding whitespace

This is so a single line comment can be used to exclude <if> in .js
files. I've added tests for both <include> and <if>. They need to be
commented for clang-format purposes.

BUG=678778
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2615953002
Cr-Original-Commit-Position: refs/heads/master@{#441853}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 42fdc659ccc288d4a3a01f202c7d667e76b04192
2 files changed