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Most mojo services are simple utility processes that don't require
blink web sandbox support. However, services such as pdf compositor
would need such support to access and load fonts. This CL does the
following two major tasks:
-- Add SandboxSupportedUtilityBlinkPlatformImpl and implements it
to allow utility process to have the sandbox support when needed;
-- Move LoadFont() interprocess call on MacOS from renderer to
content/common, so it can be used by both renderer and utility
processes.

BUG=809763,813034

Change-Id: I4f8b50cc5cc31ac68dfbc2ba5b1195fb4e0748cc
Reviewed-on: https://chromium-review.googlesource.com/919968
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542361}