Add option to specify font fallback when creating the skia font manager

Add option to specify font fallback when creating the skia font manager

This originally existed as https://codereview.chromium.org/1740533003/,
but then got reverted for causing perf problems at startup. This change
avoids that by allowing callers to specify their own font fallback, and
only uses the system fallback if a fallback is needed and none was
provided.

This is part 1 of a three part change.
1: https://codereview.chromium.org/1878843002/
   Adds support for specifying a font fallback in skia
2: https://codereview.chromium.org/1846433005/
   Implements the fallback proxy in Chromium
3: https://codereview.chromium.org/1883483002/
   Adds code to blink to call skia's fallback API

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Review URL: https://codereview.chromium.org/1878843002