Fix a couple minor nits with command buffer handling in ANGLE:
1. Make initialize() common CommandBufferHelper function
2. Reduce some common code when starting renderpass

Bug: b/154030403
Change-Id: Ie9cfcd7c053c8605b16bc2dee7f8d69418922492
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2175106Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>