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The commandline param --gaia-config allows specifying a json file that
overrides various Gaia-related URLs. This CL adds support for
overriding the Sync URL via the same file.

Bug: 1129493
Change-Id: Ieb1fee4b32f247acb69fde659c2c0c9564bcb130
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418334Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808498}