Do not require WebView usage to come from main thread

So long as all usage is from a single thread per instance it is OK if
it's a non-main thread.
(But note that for apps targeting JB MR2 this was enforced to be the
main thread anyway).

Bug 10937207

Change-Id: Ibc2496d5cef97b4685e001086f712fcaac231024
1 file changed