Take mutex when calling into FreeType.

SkScalerContext_FreeType::generateCharToGlyph and generateGlyphToChar
do not take a mutex when calling into FreeType, but they need to.
The setupSize method also requires the mutex to be locked, which is
not a problem since it currently always is, but add a debug assert
to ensure that it continues to be.

BUG=chromium:542640

Review URL: https://codereview.chromium.org/1431683006
1 file changed