This patch implements a mechanism for more granular link URL permissions (filtering on scheme/host). This fixes the bug that allowed PDFs to have working links to any "chrome://" URLs.

BUG=528505,226927

Review URL: https://codereview.chromium.org/1362433002

Cr-Commit-Position: refs/heads/master@{#351705}
bool GetGuestCallback(content::WebContents** guest_out,
                      content::WebContents* guest) {
  EXPECT_FALSE(*guest_out);
  *guest_out = guest;
  return false;
}

bool GetGuestCallback(content::WebContents** guest_out,
                      content::WebContents* guest) {
  EXPECT_FALSE(*guest_out);
  *guest_out = guest;
  return false;
}
