Small crash fix for set pages dialog drag'n'drop issue.

R=thakis@chromium.org
BUG=302284

Review URL: https://codereview.chromium.org/52823006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232971 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232971
Simplify rate limiting since it's main thread shared context only

This simplifies the compositor rate limiting code to only rate limit the
shared main thread context, which as it happens is the only context that uses
this feature at all.

BUG=181120

Review URL: https://codereview.chromium.org/53153006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232972 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232972