Fix unsigned/signed comparison in VertexDataManager.

Affects Debug/Win32 only.

BUG=angleproject:1136

Change-Id: I29fbf06077014c313489be07ff983009afb652fd
Reviewed-on: https://chromium-review.googlesource.com/296362
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/VertexDataManager.cpp b/src/libANGLE/renderer/d3d/VertexDataManager.cpp
index 254fc96..559e633 100644
--- a/src/libANGLE/renderer/d3d/VertexDataManager.cpp
+++ b/src/libANGLE/renderer/d3d/VertexDataManager.cpp
@@ -265,7 +265,7 @@
             size_t totalCount = ComputeVertexAttributeElementCount(attrib, count, instances);
             ASSERT(!bufferImpl ||
                    ElementsInBuffer(attrib, static_cast<unsigned int>(bufferImpl->getSize())) >=
-                       totalCount);
+                       static_cast<int>(totalCount));
 
             gl::Error error = mStreamingBuffer->reserveVertexSpace(
                 attrib, static_cast<GLsizei>(totalCount), instances);