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As focusable() may effect the preferred size it is important that it
returns the last value passed to SetFocusable() rather than if the
view is focusable right now. Otherwise focusable() could change value
based on enabled or visible state, resulting in the preferred size
bouncing around.

I'm also making focusable() protected as it's really only subclasses
that should care about it. Especially since we also have
IsFocusable().

BUG=330696
TEST=covered by test
R=ben@chromium.org

Review URL: https://codereview.chromium.org/129863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244282 0039d316-1c4b-4281-b951-d872f2087c98