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Returns DocumentEntry::EntryKind for corresponding MIME type of the
entry.

This CL is split off of https://chromiumcodereview.appspot.com/10837201/

BUG=chromium:127728
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10828277

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151425 0039d316-1c4b-4281-b951-d872f2087c98