Fix sandbox ASan test to cope with forward slashes in __FILE__

sandbox/win/src/address_sanitizer_test.cc

Index: sandbox/win/src/address_sanitizer_test.cc
diff --git a/sandbox/win/src/address_sanitizer_test.cc b/sandbox/win/src/address_sanitizer_test.cc
index a18cab7ab62fc5bd8fac5e34dad58c14348d81ef..18239126188c5672d2439fb3e32a859af42f72cd 100644
--- a/sandbox/win/src/address_sanitizer_test.cc
+++ b/sandbox/win/src/address_sanitizer_test.cc
@@ -93,7 +93,10 @@ TEST_F(AddressSanitizerTests, TestAddressSanitizer) {
         << "There doesn't seem to be an ASan report:\n" << data;
     ASSERT_TRUE(strstr(data.c_str(), "AddressSanitizerTests_Report"))
         << "The ASan report doesn't appear to be symbolized:\n" << data;
-    ASSERT_TRUE(strstr(data.c_str(), strrchr(__FILE__, '\\')))
+    std::string source_file_basename(__FILE__);
+    size_t last_slash = source_file_basename.find_last_of("/\\");
+    last_slash = last_slash == std::string::npos ? 0 : last_slash + 1;
+    ASSERT_TRUE(strstr(data.c_str(), &source_file_basename[last_slash]))
         << "The stack trace doesn't have a correct filename:\n" << data;
   } else {
     LOG(WARNING) << "Not an AddressSanitizer build, skipping the run.";