Speculative fix for crash in ScrollAnchor::Clear()

The line removed in this CL was added in:

https://chromium-review.googlesource.com/c/chromium/src/+/914864

That CL was to fix a crash, but the crash only occurs when
root layer scrolling is enabled.  The call to ScrollAnchor::Dispose
from LocalFrameView::Dispose was added just for completeness, but
does not affect the original crash.

This new crash suggests that LocalFrameView may still process scroll
events after Dispose() has been called.  Which is strange and
horrible, but probably not worth fully investigating at this point.
Instead, just remove the call to scroll_anchor_.Dispose() and wait
for root layer scrolling to make the surrounding code obsolete.

BUG=812023
R=skobes@chromium.org,bokan@chromium.org

Change-Id: I0715ab5a76e2fee6b4f378fc470d816b851bfa44
Reviewed-on: https://chromium-review.googlesource.com/919396Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536759}

third_party/WebKit/Source/core/frame/LocalFrameView.cpp

@@ -399,8 +399,6 @@ void LocalFrameView::Dispose() {
           this->GetScrollingCoordinator())
     scrolling_coordinator->WillDestroyScrollableArea(this);
 
-  scroll_anchor_.Dispose();
-
   Page* page = frame_->GetPage();
   // TODO(dcheng): It's wrong that the frame can be detached before the
   // LocalFrameView. Figure out what's going on and fix LocalFrameView to be