[Chromecast] Enable Devtools browser target

This is needed to enable remote browser traces on Chromecast.

Bug: internal b/65209777
Change-Id: I0ea49227ac484f7a278328eeff6eedfdac7bf946
Reviewed-on: https://chromium-review.googlesource.com/662362


Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501367}