Use the following names:

 - flushAndBeginRenderPass
 - endRenderPassAndGetCommandBuffer
 - flushAndGetPrimaryCommandBuffer

This clarifies that a flush is part of the method.

Bug: angleproject:4029
Change-Id: I6c870761339ea7aa39c83142200781ba39ad6a4b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2068129
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>