Prevent implicit cast from 0 to StructPtr<T>.

Change-Id: Ibe9f789de8af9717b7d9a8d1f82f024acb04d504
Reviewed-on: https://chromium-review.googlesource.com/935753
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#554262}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d2ca40e5ebf0f76aed44460e453d78227d63e7dc

nocompile.gni

@@ -28,25 +28,34 @@
 #
 # Example .nc file:
 #
-#   #if defined(TEST_NEEDS_SEMICOLON)  // [r"expected ',' or ';' at end of input"]
+#   #if defined(NCTEST_NEEDS_SEMICOLON)  // [r"expected ',' or ';' at end of input"]
 #
 #   int a = 1
 #
-#   #elif defined(TEST_NEEDS_CAST)  // [r"invalid conversion from 'void*' to 'char*'"]
+#   #elif defined(NCTEST_NEEDS_CAST)  // [r"invalid conversion from 'void*' to 'char*'"]
 #
 #   void* a = NULL;
 #   char* b = a;
 #
 #   #endif
 #
-# If we needed disable TEST_NEEDS_SEMICOLON, then change the define to:
+# If we need to disable NCTEST_NEEDS_SEMICOLON, then change the define to:
 #
-#   DISABLE_TEST_NEEDS_SEMICOLON
-#   TEST_NEEDS_CAST
+#   DISABLED_NCTEST_NEEDS_SEMICOLON
+#   NCTEST_NEEDS_CAST
 #
 # The lines above are parsed by a regexp so avoid getting creative with the
 # formatting or ifdef logic; it will likely just not work.
 #
+# To run the tests, try to build the test target.  For example:
+# - Example based on the BUILD.gn example above.
+#   $ ninja -C out ...:my_module_nc_unittests
+# - Example based on actual tests in //base:
+#   $ ninja -C out base:base_nocompile_tests
+# Building the test target will succeed if the expected compile errors happened
+# and will fail if either 1) there were no compile errors, or 2) the compile
+# errors didn't match the pattern expected by the tests.
+#
 # Implementation notes:
 # The .nc files are actually processed by a python script which executes the
 # compiler and generates a .cc file that is empty on success, or will have a
@@ -92,6 +101,7 @@ if (enable_nocompile_tests) {
         "-Wfatal-errors",
         "-Wthread-safety",
         "-I" + rebase_path("//", root_build_dir),
+        "-Igen",
       ]
       if (sysroot != "") {
         args += [