Move views event targeting into ViewTargeterDelegate::TargetForRect()

Move views event targeting into ViewTargeterDelegate::TargetForRect()

Move the default event-targeting implementation for views from
View::GetEventHandlerForRect() into the new method
ViewTargeterDelegate::TargetForRect() and make the method
View::GetEventHandlerForRect() call into its effective ViewTargeter
instead. The "effective" targeter for a View is the
ViewTargeter installed on that View if one exists, otherwise
it is the ViewTargeter installed on its root view.

Once this change has landed, the overrides of
View::GetEventHandlerForRect() can be removed and their
implementations moved into overrides of
ViewTargeterDelegate::TargetForRect().
View::GetEventHandlerForRect() can then be made
non-virtual.

BUG=391845
TEST=existing coverage in ViewTest.GetEventHandlerForRect

Review URL: https://codereview.chromium.org/401933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284342 0039d316-1c4b-4281-b951-d872f2087c98