Bug 1479240: Make system metric queries always return false instead of not matching for fingerprinting-resistance. r=heycam

Looks like these used this mechanism for no great reason, and actually doing
this exposes trivially whether fingerprinting-resistance is enabled, which looks
like an anti-goal (if a media query parses correctly, and doesn't match either 1
or 0, then fingerprinting-resistance is enabled).

Differential Revision: https://phabricator.services.mozilla.com/D2493

MozReview-Commit-ID: 76tIIkwlpeP

layout/style/nsMediaFeatures.cpp

@@ -393,8 +393,15 @@ GetTransform3d(nsIDocument* aDocument, const nsMediaFeature*,
 }
 
 static bool
-HasSystemMetric(nsAtom* aMetric)
+HasSystemMetric(nsIDocument* aDocument,
+                nsAtom* aMetric,
+                bool aIsAccessibleFromContent)
 {
+  if (aIsAccessibleFromContent &&
+      nsContentUtils::ShouldResistFingerprinting(aDocument)) {
+    return false;
+  }
+
   nsMediaFeatures::InitSystemMetrics();
   return sSystemMetrics->IndexOf(aMetric) != sSystemMetrics->NoIndex;
 }
@@ -403,27 +410,17 @@ static void
 GetSystemMetric(nsIDocument* aDocument, const nsMediaFeature* aFeature,
                 nsCSSValue& aResult)
 {
-  aResult.Reset();
-
   const bool isAccessibleFromContentPages =
     !(aFeature->mReqFlags & nsMediaFeature::eUserAgentAndChromeOnly);
 
   MOZ_ASSERT(!isAccessibleFromContentPages ||
              *aFeature->mName == nsGkAtoms::_moz_touch_enabled);
-
-  if (isAccessibleFromContentPages &&
-      nsContentUtils::ShouldResistFingerprinting(aDocument)) {
-    // If "privacy.resistFingerprinting" is enabled, then we simply don't
-    // return any system-backed media feature values. (No spoofed values
-    // returned.)
-    return;
-  }
-
   MOZ_ASSERT(aFeature->mValueType == nsMediaFeature::eBoolInteger,
              "unexpected type");
 
   nsAtom* metricAtom = *aFeature->mData.mMetric;
-  bool hasMetric = HasSystemMetric(metricAtom);
+  bool hasMetric =
+    HasSystemMetric(aDocument, metricAtom, isAccessibleFromContentPages);
   aResult.SetIntValue(hasMetric ? 1 : 0, eCSSUnit_Integer);
 }
 

layout/style/test/chrome/bug418986-2.js

@@ -104,10 +104,13 @@ var testToggles = function (resisting) {
   suppressed_toggles.forEach(
     function (key) {
       var exists = keyValMatches(key, 0) || keyValMatches(key, 1);
-      if (resisting || (!toggles_enabled_in_content.includes(key) && !is_chrome_window)) {
+      if (!toggles_enabled_in_content.includes(key) && !is_chrome_window) {
          ok(!exists, key + " should not exist.");
       } else {
          ok(exists, key + " should exist.");
+        if (resisting) {
+          ok(keyValMatches(key, 0) && !keyValMatches(key, 1), "Should always match as false");
+        }
       }
     });
 };
@@ -214,7 +217,7 @@ var generateCSSLines = function (resisting) {
       if (!toggles_enabled_in_content.includes(key) && !resisting && !is_chrome_window) {
         lines += "#" + key + " { background-color: green; }\n";
       } else {
-        lines += suppressedMediaQueryCSSLine(key, resisting ? "red" : "green");
+        lines += suppressedMediaQueryCSSLine(key, "green");
       }
     });
   if (OS === "WINNT") {