Replace ContentSecurityPolicy::client() with a method returning an ExecutionContext

As bug305497 is marked WontFix, I'd like to merge back
ExecutionContextClient to ExecutionContext for readability. This is 1st
step for that.

Note about behavior change induced by this CL

addConsoleMessage():
When addConsoleMessage() is called via ExecutionContextClient, it just
calls addMessage(). But ExecutionContext has non-virtual
addConsoleMessage() which checks if m_client is non-NULL. We'll call
the latter after this CL. If this change breaks something, that means
such an addConsoleMessage() call has been invalid since it's called
on an ExecutionContext that is already partially destructed.

isDocument():
OK for the same reason.

disableEval():
OK for the same reason.

securityContext() and reportBlockedScriptExecutionToInspector():
No change as ExecutionContext doesn't have a non-virtual override for it.

contextURL() and contextCompleteURL():
No change as they're implemented only on ExecutionContextClient.

BUG=305497

Review URL: https://codereview.chromium.org/332993002

git-svn-id: svn://svn.chromium.org/blink/trunk@176183 bbb929c8-8fbe-4397-9dbb-9b2b20218538