net: Allow URLFetcher users to inject custom URLFetcherResponseWriter

Add a new method URLFetcher::SaveResponseWithWriter()
Add NET_EXPORT to URLFetcherResponseWriter and its subclasses so that user code can inherit them to implement custom writers.

BUG=126753
TEST=net_unittests

Review URL: https://codereview.chromium.org/26230003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228885 0039d316-1c4b-4281-b951-d872f2087c98
Move WeakPtrFactory members to the end (translate)

BUG=303818

Review URL: https://codereview.chromium.org/26727005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228886 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228886