Strips leading and trailing whitespace from tooltip

Windows was automatically doing this for us before. I can't think of a
good reason not to do it every where.

BUG=331209
TEST=covered by unit test
R=varunjain@chromium.org

Review URL: https://codereview.chromium.org/113923006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243422 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243422