Fix warning about namespacing of HelloTriangle's constructor

BUG=angleproject:892

Change-Id: Ib3b4eb597964c21aba01047e77a413ce19c35855
Reviewed-on: https://chromium-review.googlesource.com/268994
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>

samples/hello_triangle/HelloTriangle.cpp

@@ -19,7 +19,7 @@
 class HelloTriangleSample : public SampleApplication
 {
   public:
-    HelloTriangleSample::HelloTriangleSample()
+    HelloTriangleSample()
         : SampleApplication("HelloTriangle", 1280, 720)
     {
     }