Synchronize access to WebViewZygote.

The onWebViewProviderChanged callback can be entered from a binder thread,
rather than the system_server main thread. This could lead to races when
managing the webview_zygote.

Test: m
Test: Turn on Multiprocess WebView, install a new WebView provider, then
      instantiate a new WebView. The new WebView should load (note that
      this is racy so may require multiple attempts to test).

Bug: 21643067
Change-Id: I28512906c38e073d4e3d39a2f2b30dcbb50c85ff