Fix tabWidth calculation.
Fix tabWidth calculation.

Currently, Font::tabWidth() adds a fudge factor of letterSpacing()
to the tab width, to make sure the rendered width of a tab is at
least that wide.  However, the current implementation effectively
sets tab stops at multiples of (base tab width + letterSpacing()),
rather than multiples of (base tab width).  Font::tabWidth() should
always return a value that will advance the position to a multiple of
the base tab width.

This should land before codereview.chromium.org/870763003

R=eae@chromium.org,timloh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/878373003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@189291 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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