Fix crash when InkDropHostView::SetHasInkDrop() is called multiple times

InkDropHostView installs an InkDropGestureHandler which owns a
ScopedTargetHandler. Multiple ScopedTargetHandler's should be nested
properly, so an InkDropGestureHandler cannot be deleted just after
another one is created.

BUG=622905

Review-Url: https://codereview.chromium.org/2096003002
Cr-Commit-Position: refs/heads/master@{#402518}
4 files changed