Rename acceleratedCompositingForFixedPosition to preferCompositingToLCDText.

This adds a preferCompositingToLCDText setting, and makes it be set by the
setAcceleratedCompositingForFixedPositionEnabled() WebSettings method so
that behaviour is not changed. Next, Chromium will be changed to call
setPreferCompositingToLCDTextEnabled instead so we can remove this method
from settings.

Later, other accelerated compositing trigger settings will be merged with
preferCompositingToLCDText.

R=abarth
BUG=365851

Review URL: https://codereview.chromium.org/476273003

git-svn-id: svn://svn.chromium.org/blink/trunk@180392 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Change the expand button to unfold-more.

This isn't awesome, but it was the best icon I could find and it
at least will be more clear since it's not using something that's
overloaded for opening menus.

BUG=404049
NOTRY=true

Review URL: https://codereview.chromium.org/475323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180393 bbb929c8-8fbe-4397-9dbb-9b2b20218538