Implementation of the woff2 format changes proposed by the W3C working group.

  * size of compressed font data is in the woff2 header
  * bits 5 and 6 of the table flag byte are reserved, must be 0
  * bit 7 of the table flag byte is the transform bit

This ports the decoder part of the font-compression-reference change:
https://code.google.com/p/font-compression-reference/source/detail?r=22e2c328288610e63c03e8de0b9c18b1b926cff6

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/216043003
Remove unused kShortFlagsContinue.

This will fix Chromium's clang build error.

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/216263002