Mutator created for bullet offset attribute in FontMetric.

See commit description in 8bfccd3a ("vcl: Create accessor
and mutator for font scaling in FontMetric") for reasoning behind
patch.

Unit tests
- check to ensure that can get and set bullet offset attribute
- check equality operator on FontMetric after setting bullet offset
  attribute

Change-Id: I87a76982a8b3ed697664299cb340fa35fb514c0e