[Offline pages] Downloads UI: Extening DownloadSharedPreferenceEntry to include offline page
Introduces DownloadSharedPreferenceEntry version 3, which:
* Adds item type to distinguish between offline page and download
* Introduces more tests for parsing and serialization
* Moves tests to jUnit.
BUG=
630817
Committed:
https://crrev.com/8ec49e50517e2c62e6de74c4159eb63032e2e9dc
Cr-Commit-Position: refs/heads/master@{#411185}