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Now, when multiple entries contain the same content, multiple
table-of-contents entries will be created, but only one data region.

As of now, en-US.pak has ~3200 entries, and 350 of them are duplicates.

For MonochromePublic.apk, shrinks uncompressed .pak size by ~130kb, and
compressed .pak size by 32kb.

BUG=738566

Review-Url: https://codereview.chromium.org/2969123002
Cr-Commit-Position: refs/heads/master@{#488215}