Make GetWeakPtr() const

Need to make this method const for its use in another const method in
RenderWidgetHostViewAura in https://crrev.com/c/2326556

Bug: 1109074
Change-Id: Ic5c6b49bd75b402233fb1ff1d98cca044cd6b5c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332081Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: My Nguyen <myy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793565}

base/memory/weak_ptr.h

@@ -324,7 +324,7 @@ class WeakPtrFactory : public internal::WeakPtrFactoryBase {
 
   ~WeakPtrFactory() = default;
 
-  WeakPtr<T> GetWeakPtr() {
+  WeakPtr<T> GetWeakPtr() const {
     return WeakPtr<T>(weak_reference_owner_.GetRef(),
                       reinterpret_cast<T*>(ptr_));
   }