Vulkan: Rename getSubmitFence to getNextSubmitFence. 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>