Remove Client= relationship in omnibox.mojom

Remove Client= relationship in omnibox.mojom

omnibox.mojom defines the following interface for javascript to request
omnibox suggestions from C++ code:

interface OmniboxUIHandlerMojo {
  StartOmniboxQuery(string input_string,
      int32 cursor_position,
      bool prevent_inline_autocomplete,
      bool prefer_keyword,
      int32 page_classification);
};

and the corresponding API for sending results back:

interface OmniboxPage {
  HandleNewAutocompleteResult(OmniboxResultMojo result);
};

These were defined as clients of each other so they shared a message
pipe. However, there was no way for the caller of StartOmniboxQuery() to
correlate calls to HandleNewAutocompleteResult() with a particular
invocation of StartOmniboxQuery(). Additionally we're getting rid of the
Client= notion in mojom.

This changes the StartOmniboxQuery API to take an additional parameter
of type OmniboxPage corresponding to a new message pipe that can receive
requests. The caller in omnibox.js allocates a new pipe for each query
(closing the previous one) and gets replies on the new pipe.

(this is a potential use case of a multiple return value syntax in mojom
 but that's just a proposal right now).

R=sky@chromium.org
BUG=451321
Committed: https://crrev.com/59ca1ee57ad1224e31a1449a6dc80796bd7b08f5
Cr-Commit-Position: refs/heads/master@{#314592}

[sky, 2015-02-04 18:05:09]

LGTM

[jamesr, 2015-02-04 18:33:19]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2015-02-04 18:33:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882883006/20001

[commit-bot: I haz the power, 2015-02-04 18:41:53]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-04 18:43:56]

Patchset 2 (id:??) landed as
https://crrev.com/59ca1ee57ad1224e31a1449a6dc80796bd7b08f5
Cr-Commit-Position: refs/heads/master@{#314592}