Fix and enable warning C4245 (signed/unsigned mismatch)

Change-Id: If48043835fcc98341a0626e3ece7d0e3f7958059
Reviewed-on: https://chromium-review.googlesource.com/260630
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cooper Partin <coopp@microsoft.com>
Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Program.cpp b/src/libANGLE/Program.cpp
index 6956f33..1cafe33 100644
--- a/src/libANGLE/Program.cpp
+++ b/src/libANGLE/Program.cpp
@@ -611,7 +611,7 @@
         }
     }
 
-    return -1;
+    return static_cast<GLuint>(-1);
 }
 
 int Program::getSemanticIndex(int attributeIndex)