Move Platform Context pointer to start of struct. This will ensure that new methods (placed at the end) don't collide with the context pointer when using a mismatched platform. BUG=angleproject:1892 Change-Id: Ice6a3ccaf3cc2a1e36a04bc217386cf096612176 Reviewed-on: https://chromium-review.googlesource.com/672147 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>