Record user actions for Ash accelerators.

Clean up the huge switch of accelerators as well:
* Move a bunch of code into Handle helpers.
* Alphabetize the anon namespace helpers.
* Clean up the OS_CHROMEOS groupings a bit.

BUG=321695
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/78763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238391 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238391