mac: Convert throbber view to use a CoreAnimation layer.
Use a keyframe animation to cycle through the throbber sprites without 
waking up the CPU.
I tested this by modifying the code to always spin the throbber. With 10 tabs
open with spinning throbbers, Activity Monitor shows that  CPU% goes from 10%
before this patch to about 0.2% with this patch, and idle wake-ups go from
about 30 (per second) to close to zero (there are things running other than the
throbbers).

BUG=19268
TEST=Throbbers should appear and behave exactly as before, but now with no
impact on the CPU.

Review URL: https://codereview.chromium.org/171603010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252450 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252450