GitLab

Moves UISourceCodeFrame to sources
Introduces UISourceCodeFrame.Plugin
Moves CSSSourceFrame from being a subclass to a plugin

Bug: 778043
Change-Id: Ia93a4bc2b26ae1a28505d2f82220ad5c21882f0b
Reviewed-on: https://chromium-review.googlesource.com/736672
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512729}