Allow the dep cycle chrome/browser/extensions <-> api_registration

The generated api_registration files use a lot of headers
in chrome/browser/extensions, creating a dependency cycle.
I see no obvious way to break the cycle so this just makes
it official.

There were also a couple of files in chrome/browser/accessibility
which were used from api registration, creating an unpleasant
dependency. That was fixed by putting those files in extensions
instead of in chrome/browser.

The missing dependencies triggered a lot of gn errors with a gn fixed
to look at generated files (gn:57).

The list is about 60 reports like:

ERROR at //out/Default/gen/chrome/browser/extensions/api/generated_api_registration.cc:7:11: Include not allowed.
          ^--------------------------------------------------------------------------------
It is not in any dependency of
  //chrome/browser/extensions/api:api_registration
The include file is in the target(s):
  //chrome/browser/extensions:extensions
which should somehow be reachable.

Bug: 655123
Change-Id: Ifc7809de17c9a426d2a6abdb41ecc42996b0d01f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529095Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#644715}

chrome/browser/BUILD.gn

@@ -4067,8 +4067,6 @@ jumbo_split_static_library("browser") {
 
   if (enable_extensions) {
     sources += [
-      "accessibility/accessibility_extension_api.cc",
-      "accessibility/accessibility_extension_api.h",
       "accessibility/animation_policy_prefs.cc",
       "accessibility/animation_policy_prefs.h",
       "autocomplete/keyword_extensions_delegate_impl.cc",

chrome/browser/extensions/BUILD.gn

@@ -18,6 +18,8 @@ jumbo_static_library("extensions") {
   sources = [
     # TODO These references to files in //chrome/browser/ should either be
     # moved to the browser target or the files moved to this directory.
+    "../accessibility/accessibility_extension_api.cc",
+    "../accessibility/accessibility_extension_api.h",
     "../ui/app_icon_loader.cc",
     "../ui/app_icon_loader.h",
     "../ui/toolbar/toolbar_actions_model.cc",
@@ -752,6 +754,9 @@ jumbo_static_library("extensions") {
   defines = []
 
   allow_circular_includes_from = [
+    # Lots of APIs use headers from the list above.
+    "//chrome/browser/extensions/api:api_registration",
+
     # TODO(loyso): Remove this circular dependency. http://crbug.com/876576.
     "//chrome/browser/web_applications/extensions",