Set request context when restoring cached CSSImageValue resource

This patch changes the resource context type of the FetchRequest issued
in CSSImageValue::restoreCachedResourceIfNeeded from
WebURLRequest::RequestContextUnspecified to
WebURLRequest::RequestContextImage.

BUG=630215

Review-Url: https://codereview.chromium.org/2168903002
Cr-Commit-Position: refs/heads/master@{#406846}
4 files changed