Clipboard API: Use ExecutionContext from LocalFrame instead of ScriptState.

Retrieving an ExecutionContext from the LocalFrame passed from
NavigatorClipboard ensures that the correct ExecutionContext for the current
clipboard operation is always used.

In cross-frame circumstances, ScriptState's ExecutionContext would be that
of the parent frame, but LocalFrame's ExecutionContext would be that of the
child frame.

This was mentioned in [1], and initially introduced in [2].

This should ensure that the incorrect (parent frame's) PermissionManager isn't
requested when determining whether a permission has been granted, or when
asking for permission.

This CL does not fix the potential for a null dereference if the
ExecutionContext is detached, as both the ExecutionContext::From and
GetExecutionContext will result in a null dereference in this case.

[1]: https://crrev.com/c/2016200/19/third_party/blink/renderer/modules/clipboard/clipboard_promise.cc#91
[2]: https://crrev.com/c/804973

Bug: 1002794
Change-Id: I340224959fb98d3112464f4955c6d88e537f6199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057006
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746506}

third_party/blink/renderer/modules/clipboard/clipboard_promise.cc

@@ -316,10 +316,11 @@ void ClipboardPromise::HandleWriteTextWithPermission(PermissionStatus status) {
 
 PermissionService* ClipboardPromise::GetPermissionService() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+  ExecutionContext* context = GetExecutionContext();
+  DCHECK(context);
   if (!permission_service_) {
     ConnectToPermissionService(
-        ExecutionContext::From(script_state_),
-        permission_service_.BindNewPipeAndPassReceiver());
+        context, permission_service_.BindNewPipeAndPassReceiver());
   }
   return permission_service_.get();
 }
@@ -333,7 +334,7 @@ void ClipboardPromise::RequestPermission(
   DCHECK(permission == mojom::blink::PermissionName::CLIPBOARD_READ ||
          permission == mojom::blink::PermissionName::CLIPBOARD_WRITE);
 
-  ExecutionContext* context = ExecutionContext::From(script_state_);
+  ExecutionContext* context = GetExecutionContext();
   if (!context)
     return;
   const Document& document = *Document::From(context);