Fix Chromium build of angle_end2end_tests

BUG=angleproject:892

Change-Id: I9922046fc9e4d82d7034405f5952263f982c6529
Reviewed-on: https://chromium-review.googlesource.com/278159
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/tests/gl_tests/ReadPixelsTest.cpp b/src/tests/gl_tests/ReadPixelsTest.cpp
index 0929ed6..bf48c55 100644
--- a/src/tests/gl_tests/ReadPixelsTest.cpp
+++ b/src/tests/gl_tests/ReadPixelsTest.cpp
@@ -116,10 +116,10 @@
             unsigned int a = static_cast<unsigned int>(pixel[3]);
 
             // Expect that all pixels which fell within the framebuffer are red
-            EXPECT_EQ(255, r);
-            EXPECT_EQ(0,   g);
-            EXPECT_EQ(0,   b);
-            EXPECT_EQ(255, a);
+            EXPECT_EQ(255u, r);
+            EXPECT_EQ(0u,   g);
+            EXPECT_EQ(0u,   b);
+            EXPECT_EQ(255u, a);
         }
     }
 }