aura: Do not create Env instance from WebContentsViewAura.
Initialize the aura::Env instance once when running unit-tests for contents and
above, instead of expecting WebContentsViewAura to create and initialize it
(which can lead to unexpected lifetime issues of some globals, more in the bug
referenced in this CL). Related changes necessary for this change:
. Some tests used to create their own PlatformEventSource. Remove those, since
now the aura::Env instance creates the default instance of PlatformEventSource.
. Prepare X11 correctly before running the tests by calling XInitThreads().
BUG=515031
Review URL: https://codereview.chromium.org/
1260083008
Cr-Commit-Position: refs/heads/master@{#341680}