Move debug_devtools to a buildflag_header.

Previously this was in the global features.gni file, and files did an ad-hoc
reference to the debug_devtools config if they needed the define. This is
fragile and will not scale as additional flags are added.

This patch introduces a Blink public features buildflag_header (in the
future I expect we'll also have a Blink-internal one in Source/). The
existing users of debug_devtools are updated to use this.

BUG=

Review-Url: https://codereview.chromium.org/2458573004
Cr-Original-Commit-Position: refs/heads/master@{#429362}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cb4ab75ac5e94b7857b70a9d6f73eefe9db47a7f

config/features.gni

@@ -34,12 +34,6 @@ declare_args() {
   # Non-SFI is not yet supported on mipsel
   enable_nacl_nonsfi = current_cpu != "mipsel"
 
-  # If debug_devtools is set to true, JavaScript files for DevTools are stored
-  # as is and loaded from disk. Otherwise, a concatenated file is stored in
-  # resources.pak. It is still possible to load JS files from disk by passing
-  # --debug-devtools cmdline switch.
-  debug_devtools = false
-
   # Enables WebRTC.
   enable_webrtc = !is_ios