[mojo] Force blink headers to only be included in blink

Since blink headers import internal classes WTF::Vector/String/HashMap
these files really should be only imported inside blink. Ensure
that the INSIDE_BLINK or BLINK_MOJO_IMPL is true in the header include.

Change-Id: Ib554ea263847886821200d646e73c17dcc696f60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5962696
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1375246}


CrOS-Libchrome-Original-Commit: 83b7fc2ecb7341531fdd346a905618f6fab06d12

Author: dtapuska

mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl

@@ -81,6 +81,12 @@
 #include "{{export_header}}"
 {%- endif %}
 
+{%- if for_blink %}
+#if !BLINK_MOJO_IMPL && !INSIDE_BLINK
+#error "File must only be imported inside blink"
+#endif
+{%- endif %}
+
 {% if enable_kythe_annotations -%}
 #ifdef KYTHE_IS_RUNNING
 #pragma kythe_inline_metadata "Metadata comment"

mojo/public/tools/bindings/mojom.gni

@@ -1366,6 +1366,9 @@ template("mojom") {
           "base/component_export.h",
         ]
       }
+      if (defined(variant) && variant == "blink") {
+        export_defines += [ "BLINK_MOJO_IMPL=1" ]
+      }
 
       generator_cpp_output_suffixes = []
       variant_dash_suffix = ""