Now that getLastSubmittedFence exists, the new name is more specific.

BUG=angleproject:2464

Change-Id: I0bce1387b95b7b7b739e311d37e35e7d162d8f2d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1595435
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>