Add test cases for cobalt::loader::FetcherCache (#1832)

Test FetcherCache with corner cases, and try to catch crash issue.

b/270993319

(cherry picked from commit 89a8945)

cobalt/loader/BUILD.gn

@@ -178,6 +178,7 @@ target(gtest_target_type, "loader_test") {
 
   sources = [
     "blob_fetcher_test.cc",
+    "fetcher_cache_test.cc",
     "fetcher_factory_test.cc",
     "fetcher_test.h",
     "file_fetcher_test.cc",
@@ -204,6 +205,7 @@ target(gtest_target_type, "loader_test") {
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/ots:ots",
+    "//url",
   ]
 
   data_deps = [ ":copy_loader_test_data" ]

cobalt/loader/fetcher_cache.h

@@ -42,6 +42,16 @@ class FetcherCache : public base::RefCountedThreadSafe<FetcherCache> {
   Loader::FetcherCreator GetFetcherCreator(
       const GURL& url, const Loader::FetcherCreator& real_fetcher_creator);
   void NotifyResourceRequested(const std::string& url);
+  size_t size() const {
+    DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+    CHECK_EQ(thread_id_, SbThreadGetId());
+    return total_size_;
+  }
+  size_t capacity() const {
+    DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+    CHECK_EQ(thread_id_, SbThreadGetId());
+    return capacity_;
+  }
 
   // To signal the imminent destruction of this object.  If everything is
   // working as expected, there shouldn't be any other reference of this object,