assistant: turn on build for chromeos

Bug: 114082902
Test: locally build
Change-Id: Id29bb95768d2a6da75b649e4a32b4e37c74a6897
Reviewed-on: https://chromium-review.googlesource.com/1226263Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591540}

chromeos/assistant/assistant.gni

+import("//build/config/chrome_build.gni")
+
 declare_args() {
   # Enable assistant features. Assistant related code is compiled only when
   # true. The default assistant implementation is a stub.
   enable_cros_assistant = is_chromeos
+}
 
+declare_args() {
   # Enable assistant implementation based on libassistant. This requires
   # enable_cros_assistant also enabled.
-  enable_cros_libassistant = false
-
-  # Enable building libassistant.so shared library from local checkout. This is
-  # for development purposes only.
-  enable_cros_libassistant_so = false
+  enable_cros_libassistant = enable_cros_assistant && is_chrome_branded
 }