Clarify error message for cross-origin getVRDisplays use

Previous error message was confusing as it only mentioned 'the method'
without giving any context for what 'the method' was.

Bug: 764447
Change-Id: Ia59c07bde9b9965674b3fee746b5152e33818d79
Reviewed-on: https://chromium-review.googlesource.com/722030
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509490}

third_party/WebKit/Source/modules/vr/NavigatorVR.cpp

@@ -31,8 +31,8 @@ namespace {
 const char kFeaturePolicyBlockedMessage[] =
     "Access to the feature \"vr\" is disallowed by feature policy.";
 
-const char kIframeBlockedOnUserGestureMessage[] =
-    "Access to the method is blocked on a user gesture in cross-origin "
+const char kGetVRDisplaysCrossOriginBlockedMessage[] =
+    "Access to navigator.getVRDisplays requires a user gesture in cross-origin "
     "embedded frames.";
 
 const char kNotAssociatedWithDocumentMessage[] =
@@ -128,8 +128,9 @@ ScriptPromise NavigatorVR::getVRDisplays(ScriptState* script_state) {
     // cross-origin iframes. To be backward compatible, we changed to require a
     // user gesture for cross-origin iframes.
     return ScriptPromise::RejectWithDOMException(
-        script_state, DOMException::Create(kSecurityError,
-                                           kIframeBlockedOnUserGestureMessage));
+        script_state,
+        DOMException::Create(kSecurityError,
+                             kGetVRDisplaysCrossOriginBlockedMessage));
   }
 
   // Similar to the restriciton above, we're going to block developers from