Remove unused IPC::Message priority.

Removes the PriorityValue enum and field from IPC::Message. This doesn't
appear to be used anywhere.

Changes the data message ctor to take a size_t data_len parameter.  This
works around an ambiguity problem with the main ctor, which has a similar
signature and would require lots of futzing with our test code to fix. To
make this work, the matching Pickle constructor is also changed to take a
size_t data_len parameter.

BUG=194304

Review URL: https://codereview.chromium.org/35643005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231330 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231330