Allow fallbackFactory to be null in RecordingImageBufferSurface.

This will mean that as long as the user doesn't call any "bad" methods
(writePixels etc) and is only used for one frame calling getPicture will
always result in a non-null SkPicture.

BUG=578252

Review URL: https://codereview.chromium.org/1879133004

Cr-Commit-Position: refs/heads/master@{#388026}
2 files changed