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This CL checks bookmark fields for validity/existence before parsing
them when migrating from directory to USS architectures
in uss_migrator.cc.

The original problematic code has been introduced in:
https://chromium-review.googlesource.com/c/chromium/src/+/1162226

Bug: 516866,871671
Change-Id: I843c62325f3e3ec37ce05783c999bdfd5f3542a0
Reviewed-on: https://chromium-review.googlesource.com/1164946
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581186}