Add docs about how to depend on mojom_platform from content

Source targets outside of Blink cannot depend on mojom_platform because
of the way the component build is setup. This CL adds a comment about
how to indirectly depend on it.

Change-Id: Id4b7727ba6d8045b95a907700bc41d5c8cbaea0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529164Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827739}

third_party/blink/public/mojom/BUILD.gn

@@ -13,6 +13,12 @@ if (is_android) {
 # This target includes all mojom interfaces which can be used from
 # renderer/platform. In particular these mojom interfaces can't use types that
 # are typemapped to a type in renderer/core.
+#
+# Source targets outside of Blink that want to depend on mojom_platform should
+# instead depend on //content/public/common, which ultimately exposes it as a
+# public dependency. This is because the non-Blink variant of the
+# mojom_platform C++ bindings is linked into the //content component and
+# //content/public/common has private dependencies there.
 mojom("mojom_platform") {
   generate_java = true
   sources = [