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Add testing for font service to test methods on FontLoader. FontLoader
will be extended to carry additional FontConfig IPC methods for font
fallback, querying render style for strike, and performing PPAPI
specific font functions. (See "FontService replacing Fonts Sandbox IPC"
design doc [1] and issue [2]).

Before doing that, let's ensure we have a framework to test these
methods. As the font service runs in a child utility process, we need a
way to ensure that it is set up for loading the custom FontConfig
configuration that we use for consistent font access in testing. To that
end, pass a command line feature enabled flag to the child process, and
adding while setting up the TestSuite.

Test basic matching, test failed matching, and test matching the
empty font name.

[1] https://docs.google.com/document/d/1dLBekS3RlcQFldeYZlzFGkuLyX53GBr_7Hbf4cUz1OE/
[2] crbug.com/839344

Bug: 849923
Change-Id: Ib3118d4a53b075725cbdc7fa89ba4f2b9eec8d5e
Reviewed-on: https://chromium-review.googlesource.com/1091754Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569387}