Fixing mojo dependencies for blink/modules/indexeddb

This splits the mojo code in two. Once small chunk of helper classes that
are mojo wrapped, and some high level code that is allowed to use mojo
itself.

This CL also makes it allowed and possible to use modules_export.h in
mojo types that map to code in blink/modules.

Without this patch gn check --check-generated reports errors like
the ones below:

ERROR at //out/Default/gen/third_party/blink/public/mojom/indexeddb/indexeddb.mojom-blink-forward.h:24:11: Include not allowed.
-#include "third_party/blink/renderer/modules/modules_export.h"
          ^--------------------------------------------------
It is not in any dependency of
  //third_party/blink/public/mojom:mojom_modules_blink
The include file is in the target(s):
  //third_party/blink/renderer/modules:modules
which should somehow be reachable.
------------------------------------------------
ERROR at //out/Default/gen/third_party/blink/public/mojom/indexeddb/indexeddb.mojom-blink.h:55:11: Include not allowed.
#include "third_party/blink/renderer/modules/indexeddb/idb_key_path.h"
          ^----------------------------------------------------------
It is not in any dependency of
  //third_party/blink/public/mojom:mojom_modules_blink
The include file is in the target(s):
  //third_party/blink/renderer/modules/indexeddb:indexeddb
which should somehow be reachable.

Bug: 938893
Change-Id: I0cd9bfb502fa4290a21e61f686b6095455eb4e86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547696
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#647200}
4 files changed