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This cl adds start_page_token to ResourceMetadataHeader and
DirectorySpecificInfo for supporting change list retrival based on
startPageToken instead of largestChangeId.

Bug: 840211
Change-Id: Ib498276b48b28dc1876fc9a70a4a1356b60ddeb9
Reviewed-on: https://chromium-review.googlesource.com/1050052Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557137}