Fix unittest build in x86_64.

Some size_t values (generally resulting from X.size() calls) were
compared to off_t values (such as utils::FileSize()) which resulted
in a compile error on x86_64 targets. This patch fixes those issues
with explicit casts in the unittests since the range of the expected
values is small enough.

Bug: 26955860
TEST=`mma` on edison-eng and brilloemulator_x86_64-eng.

Change-Id: I2d09d356e1b97ab6527b17596fe20d425da6d519
diff --git a/common/utils_unittest.cc b/common/utils_unittest.cc
index 6c4e5af..24468b9 100644
--- a/common/utils_unittest.cc
+++ b/common/utils_unittest.cc
@@ -413,7 +413,8 @@
   string contents;
   EXPECT_TRUE(utils::ReadFile(path.value(), &contents));
   EXPECT_EQ(contents, expected_contents);
-  EXPECT_EQ(utils::FileSize(path.value()), expected_contents.size());
+  EXPECT_EQ(static_cast<off_t>(expected_contents.size()),
+            utils::FileSize(path.value()));
 }
 
 TEST(UtilsTest, ConvertToOmahaInstallDate) {