Makes them consistent with the rest of the code. Will allow
the Sync class to call methods in GLImplFactory to generate
a resource Serial.

Also updates the unit test to the new design.

Bug: angleproject:4223
Change-Id: Ic5ba69c3a6a51d4b51d876c3b5e7eb7bc44a9ae8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1969060
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>