Hide knowledge of webkit::ppapi::PluginDelegate from chrome. This is part of moving ppapi implementation from webkit/plugins/ppapi to content/renderer.

Chrome used four methods on that interface for PDFs.
-DidStartLoading/DidStopLoading: I just exposed these methods from RenderViewImpl on the RenderView interface
-SetContentRestriction: I moved this functionality completely into chrome. This allows us to get rid of all the content restrictions related methods in the content API, and to move content_restriction.h out to chrome.
-SaveURLAs: exposed the internal SaveURL method on WebContents

BUG=263054
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/19800005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@213027 0039d316-1c4b-4281-b951-d872f2087c98