Provide StructTraits for a vector<SkBitmap>

To retain an existing ozone API while making it accessible from mojo, add
StructTraits conversion for a vector<SkBitmap>

BUG=620927

Review-Url: https://codereview.chromium.org/2096763005
Cr-Commit-Position: refs/heads/master@{#403501}
8 files changed