Adds a proxy advisor object to the network predictor which sends HEAD requests to the data reduction proxy (if enabled and active) to advise it of possible preconnect/preresolution opportunities.

BUG=270157

Review URL: https://codereview.chromium.org/22998005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235474 0039d316-1c4b-4281-b951-d872f2087c98
Devtools workspace feature means that a developer can lose
unsaved changes if the inspected page or browser gets closed.
This CL adds a warning dialog which is displayed in these cases by
devtools if the workspace is dirty.

The dialog is displayed by chaining a Devtools beforeunload 
handler before the content's beforeunload handler.
Plumbing is added to support said chaining.

For a more detailed overview, see comments in devtools_window.h

BUG=289552

Review URL: https://codereview.chromium.org/23835007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235475 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235475