Avoid device guard in WaitForQueueSerial()

Change ExecutionQueueBase::WaitForQueueSerial() to only acquire the
device mutex if waitSerial is newer than last submitted serial.
GetLastSubmittedCommandSerial() is atomic so the device mutex isn't need
to check it. This ensures WaitForQueueSerial() won't block on device
mutex unless it's likely needed for EnsureCommandsFlushed().

Bug: 412761228, 425472913
Change-Id: Ief1e54deff71f9f692f3e92c887a1d5b7ce070ae
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/283975
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Kyle Charbonneau <kylechar@google.com>