Add support for partial invalidates in WebView

Bug #3461349

This change also fixes two bugs that prevented partial invalidates
from working with other views. Both bugs were in our EGL implementation:
they were preventing the caller from comparing the current context/surface
with another context/surface. This was causing HardwareRenderer to always
redraw the entire screen.

Change-Id: I33e096b304d4a0b7e6c8f92930f71d2ece9bebf5