The main purpose of this change is to fix a fuzzer bug where we would
trigger undefined behaviour calling between Chrome and ANGLE. It's
not specced how virtual function calls work if the shared objects are
not directly linked together, and ANGLE and Chrome are not linked.
Replace the old class-style API with a C dispatch table.

Follow-up work will make the Platform owned by the Display instead of
using global variables, but fixing this is a bit tricky.

BUG=angleproject:1892
BUG=chromium:678870

Change-Id: Iad188bc2e50f2b5e4a03ce0de233d686f569c705
Reviewed-on: https://chromium-review.googlesource.com/441273
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>