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The layout tests have been using globals in skia to provide custom
font files. Skia would like to get rid of those globals. This change
adds a chromium-side SkFontMgr::Factory for the layout tests to use
instead.

BUG=chromium:567947,skia:2817

Review-Url: https://codereview.chromium.org/2599933002
Cr-Commit-Position: refs/heads/master@{#441404}