This adds a new FooBackend.cpp file for each backend that defines
exactly the symbols that are exported by libdawn_native's headers. THis
will allow factoring most of the compilation of dawn_native between the
shared and static library variants.
BUG=dawn:85
Change-Id: I69c808dac2fe5e8bb08356117e3997677787b08a
Reviewed-on: https://dawn-review.googlesource.com/c/3760
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
