Remove unnecessary dependency, and fix the chain

Test: No new tests. This should not affect behavior; only dependencies.

Font.h does not need to include SkGlyphCache.h, and doing so requires
a transitive dependency on external/skia/src/utils, which was not
intended. Forward declare it instead, and fix the build errors that
resulted.

Change-Id: Ifd09430bb848d51b139df0f0c06c63e7e48711eb