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Modifies commit logic to include the model type of items in commit
requests.  The model type is implicitly specified by the presence of
a sub-message in the EntitySpecifics field.

Prior to this commit, only bookmarks would populate the sync entities on
commit.  The special case for bookmarks was introduced recently, in
r265587.

Unlike the bookmarks case, the non-bookmark items send up only enough
specifics to identify the model type.  The rest of the specifics are not
set.

This commit also adds some tests of the bookmark and non-bookmark
deletion request behaviors.

BUG=365752,373859

Review URL: https://codereview.chromium.org/298503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271763 0039d316-1c4b-4281-b951-d872f2087c98