Allow glyph-0 font data caching for multi-font glyph pages.

The current implementation tries to follow the single-font glyph
page semantics and never stores font data for glyph 0.

But single font pages return null data in this case because they
cannot differentiate between uninitialized glyphs and glyph 0
(there is no per-glyph data).

We don't have this problem with multi-font pages: there is a
separate font data pointer per glyph. So we can safely cache
glyph 0 data in this case, and doing so yields a noticeable
record time improvement for some text-heavy pages on platforms
with expensive fallback paths.

R=jbroman@chromium.org,schenney@chromium.org,eae@chromium.org
BUG=392602

Review URL: https://codereview.chromium.org/381743003

git-svn-id: svn://svn.chromium.org/blink/trunk@177842 bbb929c8-8fbe-4397-9dbb-9b2b20218538