CommandBufferHelper::empty() logic is simplified and commented to make
it easier to read. Basically empty() means there is no work in the
CommandBufferHelper object. For RenderpassCommands, that is equivalent
to !started().

Bug: b/162521656
Change-Id: Ic9683b392835a677501cb81e5e8a025e031ebf93
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2330379Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Charlie Lao <cclao@google.com>