Plumbing for layout rectangle debug info. Companion to

https://code.google.com/p/chromium/issues/detail?id=314945

Patch from Chris Harrelson <chrishtr@gmail.com>

BUG=314945
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Plumbing for for layout rectangle debug information.

BUG=

Review URL: https://codereview.chromium.org/61883006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238297 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238297