Merge ExecutionContextClient and ExecutionContext

ExecutionContextClient was introduced in https://codereview.chromium.org/27311002. Now it isn't used.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/559553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181803 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mixed Content: introduce WebURLRequest::RequestContext checks.

We're currently performing mixed content checks based on Resource::Type,
which isn't granular enough to support the behavior specified in the
mixed content specification (hence the introduction of the
MixedContentBlockingTreatment enum).

This patch introduces a new static mixed content checking method based
on WebURLRequest::RequestContext, which will allow us to remove most
of the complexity of MixedContentChecking in future patches.

BUG=411184

Review URL: https://codereview.chromium.org/537983002

git-svn-id: svn://svn.chromium.org/blink/trunk@181804 bbb929c8-8fbe-4397-9dbb-9b2b20218538