Bug 1239126. Handle gl_InstanceID attribute with no location. r=jgilbert

This patch avoids a crash when querying program info when gl_InstanceID is used.

ANGLE will return -1 from GetAttribLocation("gl_InstanceID") (this location is
hardcoded in the ANGLE source) but still includes gl_InstanceID in it's attribute list.

dom/canvas/WebGLProgram.cpp

@@ -172,10 +172,12 @@ QueryProgramInfo(WebGLProgram* prog, gl::GLContext* gl)
 
         // Collect active locations:
         GLint loc = gl->fGetAttribLocation(prog->mGLName, mappedName.BeginReading());
-        if (loc == -1)
-            MOZ_CRASH("Active attrib has no location.");
-
-        info->activeAttribLocs.insert(loc);
+        if (loc == -1) {
+            if (mappedName != "gl_InstanceID")
+                MOZ_CRASH("Active attrib has no location.");
+        } else {
+            info->activeAttribLocs.insert(loc);
+        }
     }
 
     // Uniforms