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/payload_consumer/extent_writer_unittest.cc b/payload_consumer/extent_writer_unittest.cc
index efeab09..6884c0b 100644
--- a/payload_consumer/extent_writer_unittest.cc
+++ b/payload_consumer/extent_writer_unittest.cc
@@ -86,7 +86,8 @@
   EXPECT_TRUE(direct_writer.Write(bytes.data(), bytes.size()));
   EXPECT_TRUE(direct_writer.End());
 
-  EXPECT_EQ(kBlockSize + bytes.size(), utils::FileSize(path_));
+  EXPECT_EQ(static_cast<off_t>(kBlockSize + bytes.size()),
+            utils::FileSize(path_));
 
   brillo::Blob result_file;
   EXPECT_TRUE(utils::ReadFile(path_, &result_file));
@@ -153,7 +154,7 @@
   }
   EXPECT_TRUE(direct_writer.End());
 
-  EXPECT_EQ(data.size(), utils::FileSize(path_));
+  EXPECT_EQ(static_cast<off_t>(data.size()), utils::FileSize(path_));
 
   brillo::Blob result_file;
   EXPECT_TRUE(utils::ReadFile(path_, &result_file));
@@ -202,7 +203,7 @@
   ASSERT_TRUE(zero_pad_writer.Write(data.data(), bytes_to_write));
   EXPECT_TRUE(zero_pad_writer.End());
 
-  EXPECT_EQ(data.size(), utils::FileSize(path_));
+  EXPECT_EQ(static_cast<off_t>(data.size()), utils::FileSize(path_));
 
   brillo::Blob result_file;
   EXPECT_TRUE(utils::ReadFile(path_, &result_file));
@@ -251,7 +252,7 @@
   EXPECT_TRUE(direct_writer.End());
 
   // check file size, then data inside
-  ASSERT_EQ(2 * kBlockSize, utils::FileSize(path_));
+  ASSERT_EQ(static_cast<off_t>(2 * kBlockSize), utils::FileSize(path_));
 
   brillo::Blob resultant_data;
   EXPECT_TRUE(utils::ReadFile(path_, &resultant_data));