Cache the current WebPreferences on RenderViewHostImpl.

This makes lookups of a WebPreferences field fast.
In order do this, add code to listen to all preferences updates in order to recompute the cache.

BUG=390799

Review URL: https://codereview.chromium.org/373323003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284592 0039d316-1c4b-4281-b951-d872f2087c98
22 files changed