Multiple fixes to browser

- Ensure that clearing of the FLAG_DIM_BEHIND happens only once.
  Clearing FLAG_DIM_BEHIND window flag multiple time was causing
  unnecessary paint.
- Implement shouldDownloadFavicon API in Tab to optimize favicon
  download.

Change-Id: I8027565a29f2f1f6a2429a282c8727c6b4fe0d45
3 files changed