Remove synchronized from static methods in WebView.

ART uses the class object's embedded monitor to protect class
initialization - the same monitor used by "synchronized" applied to
static methods. This can cause deadlock in extremely rare cases of
preemption during WebView code loading.

Remove synchronized from all static methods in WebView, to avoid this
deadlock. In most cases the synchronized is simply unnecessary in the
Chromium-based WebView implementation; in CookieSyncManager we instead
use a separately allocated Lock object to preserve thread safety.

Bug: 27417671
Change-Id: I13049f23fc984b77a3f4c203a5c698858c64abfe