SkPDF to use aliased em bounds.

This restores previous behavior and works around a chromium issue.
The underlying issue in DirectWrite will need to be worked around
as well.

Bug: chromium:926885

Change-Id: I6a6cbd04db66066ad258df62d3c9fc8056c33c7f
Reviewed-on: https://skia-review.googlesource.com/c/189167
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
Cherry-Pick: 92170de80318c5998ffbb81d48a25d1b1d61593d
Approval: https://bugs.chromium.org/p/chromium/issues/detail?id=926885#c45
Reviewed-on: https://skia-review.googlesource.com/c/189492
Reviewed-by: Hal Canary <halcanary@google.com>