Fix clang syntax checking.

This fixes serveral clang syntax checking. 1) unused functions.
2) confict of struct and class. 3) unused variables.

BUG=chromium:230663
TEST=clang syntax checking passed.
     trybot daisy-release passed
Change-Id: I8dbfb7dd5f21739831843bc3e7fa8ec7864824b9
Reviewed-on: https://gerrit.chromium.org/gerrit/47997
Reviewed-by: Han Shen <shenhan@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
diff --git a/http_fetcher_unittest.cc b/http_fetcher_unittest.cc
index 4fb8903..e6204d3 100644
--- a/http_fetcher_unittest.cc
+++ b/http_fetcher_unittest.cc
@@ -322,6 +322,8 @@
  private:
   static void TypeConstraint(T *a) {
     AnyHttpFetcherTest *b = a;
+    if (b == 0) // Silence compiler warning of unused variable.
+      *b = a;
   }
 };