Fix regression with BufferStorage11 and latest storage.

With our fix to huge buffer allocations, an extra check was preventing
pixel pack from running on the first try.

BUG=angle:511

Change-Id: Idabafe4e204ad3b7bf2010595dc455affe93dae0
Reviewed-on: https://chromium-review.googlesource.com/198825
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>

src/libGLESv2/renderer/d3d11/BufferStorage11.cpp

@@ -389,7 +389,7 @@ void BufferStorage11::packPixels(ID3D11Texture2D *srcTexture, UINT srcSubresourc
 
     TypedBufferStorage11 *latestStorage = getLatestStorage();
 
-    if (packStorage && latestStorage)
+    if (packStorage)
     {
         packStorage->packPixels(srcTexture, srcSubresource, params);
         packStorage->setDataRevision(latestStorage ? latestStorage->getDataRevision() + 1 : 1);
@@ -474,6 +474,7 @@ void *BufferStorage11::map(GLbitfield access)
     ASSERT(!mMappedStorage);
 
     TypedBufferStorage11 *latestStorage = getLatestStorage();
+    ASSERT(latestStorage);
 
     if (latestStorage->getUsage() == BUFFER_USAGE_PIXEL_PACK ||
         latestStorage->getUsage() == BUFFER_USAGE_STAGING)