DevTools: WI.SourceCodeDiff should not depend on UISourceCode

WI.SourceCodeDiff is simply a text editor decorator; it should rely solely
on text editor events to updates itself.

BUG=none
R=dgozman, einbinder

Review-Url: https://codereview.chromium.org/2149203003
Cr-Commit-Position: refs/heads/master@{#405757}
2 files changed