Add "//content/common:mojo_bindings" to the deps of "//content/child"

to prevent build errors by not generated mojom headers.

Change-Id: I9fd512f6fcf1f77dcc3af9772af12bbf5e9f71c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451931Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815268}

content/child/BUILD.gn

@@ -76,6 +76,7 @@ target(link_target_type, "child") {
     "//components/webcrypto",
     "//content/app/resources",
     "//content/common",
+    "//content/common:mojo_bindings",
     "//content/public/common:common_sources",
     "//crypto:platform",
     "//device/gamepad/public/cpp:switches",