Bug 1537955 - Do not spoof DevicePixelRatio for pdfs r=Ehsan

pdfs should not have a way to exfiltrate the true device pixel ratio,
so it is safe to provide the correct value, enabling them to be rendered
correctly.

Differential Revision: https://phabricator.services.mozilla.com/D39994

--HG--
extra : moz-landing-system : lando

dom/base/nsGlobalWindowOuter.cpp

@@ -3736,7 +3736,18 @@ double nsGlobalWindowOuter::GetDevicePixelRatioOuter(CallerType aCallerType) {
   }
 
   if (nsContentUtils::ResistFingerprinting(aCallerType)) {
-    return 1.0;
+    // Spoofing the DevicePixelRatio causes blurriness in some situations
+    // on HiDPI displays. pdf.js is a non-system caller; but it can't
+    // expose the fingerprintable information, so we can safely disable
+    // spoofing in this situation. It doesn't address the issue for
+    // web-rendered content (including pdf.js instances on the web.)
+    // In the future we hope to have a better solution to fix all HiDPI
+    // blurriness...
+    nsAutoCString origin;
+    nsresult rv = this->GetPrincipal()->GetOrigin(origin);
+    if (NS_FAILED(rv) || origin != NS_LITERAL_CSTRING("resource://pdf.js")) {
+      return 1.0;
+    }
   }
 
   float overrideDPPX = presContext->GetOverrideDPPX();