[Android WebView] Remove unused mAdapterLock.

mAdapterLock was meant to be used for glue adapter instances stored in
WebViewChromiumFactoryProvider but using such a lock together with a
lock for Chromium initialization caused deadlocks. In this CL we remove
mAdapterLock since it's no longer used (either upstream or downstream).

Bug: 807332
Change-Id: I7b736942ae7fcbdaefbf888c36d84228154706b8
Reviewed-on: https://chromium-review.googlesource.com/951782
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Gustav Sennton <gsennton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541151}