Calculate the visible sound level adaptively.

Previously the sound-level indicator directly uses the sound level
from the webui and converts it the visible radius simply linearly
with static limits.

This doesn't work well with various environments.
This CL introduces a simple heuristics to calculate the limits adaptively.
- minimum level is updated when the speech hasn't started yet
- maximum level is updated when the speech has started

Even though heuristics is simple, it seems working better than
the existing static limits.

Also, this calculation is not views-specific, so it's moved to 
SpeechUIModel. It now outputs a virtual level in [0, 255] and then,
SpeechView converts the value into the actual radius.

BUG=none
R=xiyuan@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/115183003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241224 0039d316-1c4b-4281-b951-d872f2087c98
Remove code only used by Mac single threaded compositing.

Mac single threaded compositing is no longer a shipping configuration,
so remove code added to throttle swap acks for the single threaded
compositor path. The Mac single threaded path will still work, but will
not have a throttled framerate anymore. This will become even less
relevant as the Mac moves to a browser compositor.

Pull out all code associated with AboutToWaitForBackingStoreMsg,
since does nothing now.

Also remove some dead function prototypes.

BUG=286038

Review URL: https://codereview.chromium.org/116103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241225 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241225