Move GetWebkitPrefs from WebContentsImpl to RenderViewHostImpl.

GetWebkitPrefs needs to move away from WebContentsImpl, so InterstitialPageImpl can remove its dependency on WebContentsImpl. Also, it doesn't use anything specific to WebContents, but it is for a RenderViewHost, so it makes sense to move there.

BUG=304341

Review URL: https://codereview.chromium.org/45313003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231358 0039d316-1c4b-4281-b951-d872f2087c98
GTTF: make sharding_supervisor.py use --brave-new-test-launcher

- browser tests use parallel jobs by default (except interactive_ui_tests)
- sharding + PRE_ tests are handled correctly
- .isolate files updated

BUG=236893
R=maruel@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/46023003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231359 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231359