Retrieve the 'unit per EM' value from font's ascender/descender metrics.
Retrieve the 'unit per EM' value from font's ascender/descender metrics.

- In some font files (e.g. Roboto-Regular.ttf), an 'unit per EM' value is
  not set as expected (expected to be equal to 'ascender' - 'descender').
  With the change FontManager derives the value from 'ascender' - 'descender'
  rather than using a value in a font file.

Change-Id: Ia6c985cc91b0b814e32307215460042545ad3165
Tested: on OSX.

google/flatui,google/flatui,google/flatui,google/flatui