[webkit] Postpone sending scroll message to webcore thread

Post a message to the UI thread instead, to send the
scroll message later. Thus reduce the computation until
currnet scrolling finishes

Change-Id: Iff94af37e9070e63e0797195c7588343ab077bb3
[webkit]Change the thread priority

Change the values used to dynamically modify thread priority to
improve scrolling and pinc to zoom use cases.

Change-Id: I861b3042079058fdd614390c563f23d3cab3acd9
[webkit] Runtime enablement of WebGL

Hook up debug setting to enable/disable WebGL.

(cherry-picked from commit 69ee15e5b0bc1e0b2d41d80f9f5247b826c62ceb)

Change-Id: Ib07427378450a858d6b6308e23b5e1aaac002dd3