Clean up and refactor CompositorFrame fuzzer helper methods.

Refactor the task of constructing a CompositorFrame into a builder
class, which will make it easier going forward to modularize the
utility code while keeping state such as allocated memory.

Additionally, a few other usability changes (adding optional logging
and making some protobuf fields optional with sensible defaults).

R=kylechar@chromium.org, riajiang@chromium.org

Bug: 923088
Change-Id: I10810b9bbf00bc5c5fb25cd3e418597279d7e047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580045
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Céline O'Neil <celineo@google.com>
Auto-Submit: Céline O'Neil <celineo@google.com>
Cr-Commit-Position: refs/heads/master@{#654105}
4 files changed