Make blink_headers depend on media_devices_mojo_bindings

Media devices mojo bindings are not generated for targets that depend
on blink_headers causing compilation failures.
This CL adds missing dependency for blink_headers.

blink_headers --> generate_mojo_bindings --> media_devices_mojo_bindings

Bug: 764680
Change-Id: Idaa73dc237b93d16b3d11038e5a7ebdbb101bfdd
Reviewed-on: https://chromium-review.googlesource.com/954805
Commit-Queue: Chandan Padhi <c.padhi@samsung.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545191}

third_party/WebKit/public/BUILD.gn

@@ -943,6 +943,8 @@ group("generate_mojo_bindings") {
     # variant of the generated files.
     ":core_mojo_bindings__generator",
     ":core_mojo_bindings_blink__generator",
+    ":media_devices_mojo_bindings__generator",
+    ":media_devices_mojo_bindings_blink__generator",
     ":mojo_bindings__generator",
     ":mojo_bindings_blink__generator",
     ":offscreen_canvas_mojo_bindings_blink__generator",