Fix deps for some mojo tests

gn check --check-generated revealed a hidden dependency not
expressed in the build scripts which is fixed by this.
The exact error was like the one below (and a couple like it):

ERROR at //out/Default/gen/mojo/public/interfaces/bindings/tests/test_import.mojom-blink-test-utils.cc:23:11: Can't include this header from here.
          ^--------------------------------------------------------------
The target:
  //mojo/public/interfaces/bindings/tests:test_exported_import_blink
is including a file from the target:
  //mojo/public/interfaces/bindings/tests:test_export_blink__generator

It's usually best to depend directly on the destination target.
In some cases, the destination target is considered a subcomponent
of an intermediate target. In this case, the intermediate target
should depend publicly on the destination to forward the ability
to include headers.

Dependency chain (there may also be others):
  //mojo/public/interfaces/bindings/tests:test_exported_import_blink -->
  //mojo/public/interfaces/bindings/tests:test_export_blink_component --[private]-->
  //mojo/public/interfaces/bindings/tests:test_export_blink -->
  //mojo/public/interfaces/bindings/tests:test_export_blink__generator


Bug: 938893
Change-Id: Id7d4be31ef01e37054c381a7fce7eca43ac1b753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1545914
Auto-Submit: Daniel Bratell <bratell@opera.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#647342}