Fixed compiler warning C4267 'conversion from 'size_t' to 'type', possible loss of data'

BUG=angleproject:1120

Change-Id: I01ef10bea7f487c2b394d030c76628f38d2ea645
Reviewed-on: https://chromium-review.googlesource.com/292780
Tested-by: Cooper Partin <coopp@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/IndexDataManager.cpp b/src/libANGLE/renderer/d3d/IndexDataManager.cpp
index c7c865b..1605590 100644
--- a/src/libANGLE/renderer/d3d/IndexDataManager.cpp
+++ b/src/libANGLE/renderer/d3d/IndexDataManager.cpp
@@ -222,7 +222,8 @@
             }
             ASSERT(bufferData != nullptr);
 
-            unsigned int convertCount = buffer->getSize() >> srcTypeInfo.bytesShift;
+            unsigned int convertCount =
+                static_cast<unsigned int>(buffer->getSize()) >> srcTypeInfo.bytesShift;
             error = StreamInIndexBuffer(staticBuffer, bufferData, convertCount,
                                         srcType, dstType, nullptr);
             if (error.isError())