Tab close buttons can only be targeted with touch if the whole button is visible

Tab close buttons can only be targeted with touch if the whole button is visible

As a result of r246686 landing, tabs can be too easy to close with
touch in stacked tab mode when rect-based targeting is enabled.
This patch changes the behaviour so that:

(1) Hit tests in response to a mouse event will always consider the
    region of the tab close button that is visible.
(2) Hit tests in response to a gesture will only consider the tab
    close button to be a valid target if the entire button is
    visible to the user.

Note that if there is any vertical clipping of the tab
close button, we do not consider the clipped
region to be a part of the button's bounds.

BUG=338516,332334
TEST=TabStripTest.ClippedTabCloseButton, also manually tested on Windows when tab close buttons are vertically clipped

Review URL: https://codereview.chromium.org/147623004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249331 0039d316-1c4b-4281-b951-d872f2087c98