If renderpass has started but no command has been issued, we should
chose to use loadOp to do clear instead of clearWithClearAttachment.

Bug: b/162521656
Change-Id: Ia681fc96442c7916b4b92c48e970d9ed239a28c8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2330225
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>