Directly access NetworkDelegate in URLRequestContext getter.

This change is motivated by a desire to remove external accesses to a request 
context's delegate. (If a block of code needs to use a NetworkDelegate, it 
should not query the URLRequestContext for it, but instead should already have 
access to whatever delegate the context uses.) 

BUG=146587


Review URL: https://chromiumcodereview.appspot.com/12755002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@187832 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed