Remove call to getMetrics from GlyphGenerator.

GlyphGenerator::generatePath does not need to call getMetrics before
calling getPath. This is wasted effort, especially since the
SkScalerContext is being used directly instead of going through a glyph
cache (which would require this ordering).

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4620

Change-Id: I140a6715a1d1692b161ee3dae48e5482b6004b63
Reviewed-on: https://skia-review.googlesource.com/4620
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>