Fix and enable warning C4244 (Conversion from 'type1' to 'type2', possible loss of data)

Change-Id: Id0e06d7d6600344d858f00dabc219d79289bbc82
Reviewed-on: https://chromium-review.googlesource.com/265020
Tested-by: Minmin Gong <mgong@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/VertexDataManager.cpp b/src/libANGLE/renderer/d3d/VertexDataManager.cpp
index cb70b9e..91770dc 100644
--- a/src/libANGLE/renderer/d3d/VertexDataManager.cpp
+++ b/src/libANGLE/renderer/d3d/VertexDataManager.cpp
@@ -291,7 +291,7 @@
     if (directStorage)
     {
         outputElementSize = ComputeVertexAttributeStride(attrib);
-        streamOffset = attrib.offset + outputElementSize * firstVertexIndex;
+        streamOffset = static_cast<unsigned int>(attrib.offset + outputElementSize * firstVertexIndex);
     }
     else if (staticBuffer)
     {