Add ExtensionUnpacker mojo, used to (unzip and) unpack a chrome
extension, and expose it to the browser via the utility process
policy file. Make callers use a utility process mojo client.

Add [Native] IPC enum param traits via a mojo typemap to handle
extensions::Manifest::Location. Set the wire limit for the enum
value to one less than Manifest::NUM_LOCATIONS (for compat with
the pre-existing custom traits removed in this patch since this
patch auto-generates them: manifest_location_param_traits.cc).

In the extensions/utility/utility_handler.cc, retain the CHECKs
on the Manifest::Location enum limits while bug 692069 is still
being investigated.

Move DecodeImages declarations from the extension messages-file
into its own file (extension_utility_types.h) to decouple those
declarations from this messages-file. A follow-up patch deletes
the messages-file so DecodeImages needs a new home.

Update all BUILD.gn for using clients to directly depend on the
//extensions/common:common target (and hence, its :mojo) to fix
post-compile step failures (warning about under-specified BUILD
files via "targets is_dirty" errors, see review comment #169).

Add TestContentBrowserClient to the extension test harness, and
also an overlay file in test/data (needed to expose the mojo in
the extension test harness to the unit tests listed below).

Covered by existing unit tests:
  {sandboxed_unpacker, zipfile_installer}_unittest.cc

BUG=691410

Review-Url: https://codereview.chromium.org/2697463002
Cr-Commit-Position: refs/heads/master@{#457374}