[PDF] Fix font width generation when glyph 0 is used.

Using glyph 0 caused gid 0 to be in the subset list twice, which violated an assumption in the code.  Added an assert for the assumption and updated the code to  not insert gid 0 into the subset list twice.

BUG=skia:1889
R=bungeman@google.com

Author: vandebo@chromium.org

Review URL: https://codereview.chromium.org/113093004

git-svn-id: http://skia.googlecode.com/svn/trunk@12632 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed