cc: Plumbing for impl thread micro benchmarks

This patch adds plumbing for running micro benchmarks on the impl
thread. The concept is as follows:

- Create a main thread benchmark via gpu benchmarking extension
- On commit complete, if the benchmark provides an impl thread
  benchmark, it is created and scheduled to be run on the impl thread
- Impl thread benchmarks are run after an appropriate callback
- Impl thread benchmarks post a done callback (with results) that is
  run on the main thread
- The main thread benchmark gets the callback and posts its own done
  callback (with aggregated results)
- This gets back to the gpu benchmarking extension which sends the
  response back to the caller

R=enne@chromium.org, nduca@chromium.org

Review URL: https://codereview.chromium.org/67563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235585 0039d316-1c4b-4281-b951-d872f2087c98
[rAC, OSX] Use NSTextAttachement for icon

Second part of switching to a single NSTextView for suggestion text.
This gets rid of a separately displayed icon and instead handles it
as an NSTextAttachment inside the NSTextView.

R=rsesek
BUG=277125

Review URL: https://codereview.chromium.org/67813003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235586 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235586