Moving the Platform Methods init to GetDisplay allows us to override
the platform methods as soon as possible. This lets Chromium override
the methods via ANGLEGetDisplayPlatform. It also lets us reset the
platform methods when making a new display. Note that because the
methods pointer is global it's a bit tricky to manage between swapping
configs in ANGLE tests. Using a separate test process for each config
should help.

Bug: angleproject:3393
Change-Id: I351f86422e53075563f6d8fce32efbd40023cd23
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592045
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>