[Android WebView] Provide a response header for application-provided content

Add "Client-Via: shouldInterceptRequest" header to responses created
for the content provided by the embedder via WebViewClient.shouldInterceptRequest.

This helps easily identifying application-provided responses in DevTools
Network tab.

BUG=347503

Review URL: https://codereview.chromium.org/177423009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253836 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253836
Delete images are no longer be used as a result of https://codereview.chromium.org/178193004/

BUG=297187
TEST=None
TBR=jamescook

Review URL: https://codereview.chromium.org/182313008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253837 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253837