Bug 1185815 - Hoist generation increment. r=jgilbert

So 'default' WebGL objects are in the same generation as the current context.

dom/canvas/WebGLContext.cpp

@@ -901,6 +901,11 @@ WebGLContext::SetDimensions(int32_t signedWidth, int32_t signedHeight)
         return NS_ERROR_FAILURE; // exit without changing the value of mGeneration
     }
 
+    // increment the generation number - Do this early because later
+    // in CreateOffscreenGL(), "default" objects are created that will
+    // pick up the old generation.
+    ++mGeneration;
+
     // Get some prefs for some preferred/overriden things
     NS_ENSURE_TRUE(Preferences::GetRootBranch(), NS_ERROR_FAILURE);
 
@@ -949,9 +954,6 @@ WebGLContext::SetDimensions(int32_t signedWidth, int32_t signedHeight)
     mResetLayer = true;
     mOptionsFrozen = true;
 
-    // increment the generation number
-    ++mGeneration;
-
     // Update our internal stuff:
     if (gl->WorkAroundDriverBugs() && gl->IsANGLE()) {
         if (!mOptions.alpha && gl->Caps().alpha)