Reland of Add a NavigationThrottle to the public content/ interface

Reland of Add a NavigationThrottle to the public content/ interface (patchset #1 id:1 of https://codereview.chromium.org/1350913008/ )

Reason for revert:
Test failure on linux_chromium_chromeos_rel_ng is fixed, but to be sure (e.g., it's not a problem with test bot?) let's see if bug reappears when we undo the revert.

Original issue's description:
> Revert of Add a NavigationThrottle to the public content/ interface (patchset #18 id:350001 of https://codereview.chromium.org/1269813002/ )
> 
> Reason for revert:
> linux_chromiium_chromeos_rel_ng => browser_test => SelectFileDialogExtensionBrowserTest seems to faile a lot, testing.
> 
> Original issue's description:
> > Add a NavigationThrottle to the public content/ interface
> >
> > This CL adds a NavigationThrottle class to the public content/ interface. A NavigationThrottle is used to control the flow of navigations. It lives entirely on the UI thread. Eventually, all components that want the functionality of a ResourceThrottle for navigations should transition to a NavigationThrottle: the new architecture for navigations (browser-side navigation) will not support ResourceThrottles for main resources load. See https://docs.google.com/document/d/1ICLLQoC9EsZ-bWH4ZKRhPCIoZKn6pOj02SlGl6SKH6Y/edit?pli=1#heading=h.fmxjmgvbgg7x for the design doc.
> >
> > This Cl also transition the InterceptNavigationresourceThrottle to the Navigationthrottle model.
> > BUG=504347
> >
> > Committed: https://crrev.com/394057986ff5d00a841a976f3bd8d599603acaa1
> > Cr-Commit-Position: refs/heads/master@{#350092}
> 
> TBR=nasko@chromium.org,davidben@chromium.org,carlosk@chromium.org,fdegans@chromium.org,avi@chromium.org,creis@chromium.org,nick@chromium.org,asargent@chromium.org,sky@chromium.org,mnaganov@chromium.org,clamy@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=504347
> 
> Committed: https://crrev.com/2ad8595f7a925851acd3ae733f359fafe6223ff5
> Cr-Commit-Position: refs/heads/master@{#350157}

TBR=nasko@chromium.org,davidben@chromium.org,carlosk@chromium.org,fdegans@chromium.org,avi@chromium.org,creis@chromium.org,nick@chromium.org,asargent@chromium.org,sky@chromium.org,mnaganov@chromium.org,clamy@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=504347

[huangs, 2015-09-22 16:37:06]

Created Reland of Add a NavigationThrottle to the public content/ interface

[huangs, 2015-09-22 17:19:05]

Confirmed that the same error would return if we reland the CL.  Closing this
CL.