Introduce content::PageState.

This is a concrete class wrapping a string that contains the
data of a serialized WebKit::WebHistoryItem class.  Previously,
we've just passed around these as strings, giving them names
like "state", "content_state" or "history_state".  It has been
hard to identify all of the places in the code where these
strings get passed around.  A concrete class should make usage
more apparent.  Plus, instead of manipulating the strings using
methods from webkit/glue/glue_serialize.h, we can just declare
methods on the PageState class.  This makes the code much cleaner.

This first pass just implements PageState in terms of glue_serialize.
It also adds content/public/renderer/history_item_serialization.h
as the home for PageState to WebKit::WebHistoryItem conversion,
which should ideally only be usable from the renderer process.
(This bit is a step toward resolving bug 237243.)

page_state.h declares operator==() to support DCHECK_EQ, which
seems consistent with the idea of PageState being a replacement
for std::string.  I didn't want to litter tests with calls to
PageState::ToEncodedData().  That would get cumbersome.

BUG=240426
R=brettw@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/14985014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202188 0039d316-1c4b-4281-b951-d872f2087c98

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