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This CL is a follow up to https://crbug.com/1016615, and stores the
users chosen font magnification size as a preference.

AXRelnotes: N/A

Bug: 1016615
Change-Id: I4a2331edf7d36fa12a5e260fef863cd495d90bdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212222Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799381}