[android_webview] InterceptedRequestData refactoring.

Moving the OpenInputStream, GetMimeType and GetCharset methods to the
InterceptedRequestData class from InterceptedRequestDataImpl makes the
CreateJobFor method slightly cleaner.

BUG=None
TEST=AndroidWebViewTest
Android-only change, trybots happy
NOTRY=true

Review URL: https://codereview.chromium.org/65043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234796 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234796