Added required casts to fix 64-bit compilation errors

Change-Id: I5ec9090e0129c778de8fcdc6cf70ea0fe460a19d
Reviewed-on: https://chromium-review.googlesource.com/244510
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/Context.cpp b/src/libANGLE/Context.cpp
index 81910ff..81c6598 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -230,7 +230,7 @@
 {
     GLuint handle = mResourceManager->createFenceSync();
 
-    return reinterpret_cast<GLsync>(handle);
+    return reinterpret_cast<GLsync>(static_cast<uintptr_t>(handle));
 }
 
 GLuint Context::createVertexArray()