[DL] Add lockedElement attribute to DisplayLockContext.

The locked element attribute is useful on the context since it creates
an easy access point to add elements into.

R=chrishtr@chromium.org

Bug: https://github.com/chrishtr/display-locking/issues/30
Change-Id: I726a1db72373bdde261a808b38ccb17424a21348
Reviewed-on: https://chromium-review.googlesource.com/c/1336507Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608792}

third_party/blink/renderer/core/display_lock/display_lock_context.cc

@@ -164,6 +164,10 @@ DisplayLockSuspendedHandle* DisplayLockContext::suspend() {
   return new DisplayLockSuspendedHandle(this);
 }
 
+Element* DisplayLockContext::lockedElement() const {
+  return element_;
+}
+
 void DisplayLockContext::ProcessQueue() {
   SCOPED_LOGGER(__PRETTY_FUNCTION__);
   // It's important to clear this before running the tasks, since the tasks can

third_party/blink/renderer/core/display_lock/display_lock_context.h

@@ -80,6 +80,7 @@ class CORE_EXPORT DisplayLockContext final
   // JavaScript interface implementation.
   void schedule(V8DisplayLockCallback*);
   DisplayLockSuspendedHandle* suspend();
+  Element* lockedElement() const;
 
   // Lifecycle observation / state functions.
   bool ShouldLayout() const;

third_party/blink/renderer/core/display_lock/display_lock_context.idl

@@ -15,4 +15,7 @@ interface DisplayLockContext {
   // calls to suspend are allowed: in order to resume the context, resume() must
   // be called on each of the returned DisplayLockSuspendedHandles.
   DisplayLockSuspendedHandle suspend();
+
+  // Represents the element on which this locked has been acquired.
+  [SameObject, Affects=Nothing] readonly attribute Element lockedElement;
 };