Pass GlyphBounds struct into SimpleShaper

Change SimpleShaper to pass an optional GlypBounds struct instead of
conditionally computing and populating the bounds directly on the
shaper based on the accountForGlyphBounds parameter.

BUG=418570
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/619443002

git-svn-id: svn://svn.chromium.org/blink/trunk@183014 bbb929c8-8fbe-4397-9dbb-9b2b20218538