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/http_fetcher_unittest.cc b/common/http_fetcher_unittest.cc
index 91f85db..aaa538c 100644
--- a/common/http_fetcher_unittest.cc
+++ b/common/http_fetcher_unittest.cc
@@ -923,7 +923,7 @@
                const string& url,
                const vector<pair<off_t, off_t>>& ranges,
                const string& expected_prefix,
-               off_t expected_size,
+               size_t expected_size,
                HttpResponseCode expected_response_code) {
   MultiHttpFetcherTestDelegate delegate(expected_response_code);
   delegate.fetcher_.reset(fetcher_in);