Directly compute glyphToUnicode array in DirectWrite.

Directly compute glyphToUnicode array in DirectWrite.

This new code is still rather bad, but has fewer steps.

The motivation for looking at this is that on Windows 8 the previous code
was occasionally asserting in such a way as to imply that GetGlyphIndices
might return different data the second time through the font. This avoids
that particular issue by only going through the font once.

R=mtklein@google.com, halcanary@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/472333003