Refactor ResourceSerialFactory::issueSerial so that the ASSERT is valid
in a threaded environment. Otherwise it's possible for another thread to
increment mCurrentUniqueSerial in the middle of the ASSERT.

Bug: b/168744561
Change-Id: I7208e8be91b21c50ad4a260d02d69f5b7dab2efe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2426685Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>