Move the default constructor for PlatformMethods struct from
Platform.cpp into Platform.h. This allows other projects to include
Platform.h and use PlatformMethods without having to link against ANGLE

Bug: angleproject:2528
Change-Id: Icb4b9e149e78e9ec7be85804d711d8a2216e61c4
Reviewed-on: https://chromium-review.googlesource.com/1115212
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>