Add MB config and installer target for an official CrOS builder.

This adds an MB config for 'Linux ChromeOS Buildspec Tests' builder, and makes the 'linux_packages_all' target be defined for CrOS builds as well as desktop Linux, though I have no idea if this'll actually build in that config.

TBR=dnj@chromium.org, brettw@chromium.org
BUG=643238

Review-Url: https://codereview.chromium.org/2399883003
Cr-Commit-Position: refs/heads/master@{#423739}

chrome/installer/BUILD.gn

@@ -5,7 +5,7 @@
 import("//build/config/chrome_build.gni")
 
 declare_args() {
-  enable_linux_installer = is_desktop_linux && is_chrome_branded
+  enable_linux_installer = is_linux && is_chrome_branded
 }
 
 # Meta-target that forwards to the installer of the correct type (if any).

tools/mb/mb_config.pyl

@@ -10,6 +10,7 @@
     'chromeos.chrome': {
       'amd64-generic Trusty (Informational)': 'cros_chrome_sdk',
       'Chrome4CROS Packages': 'chromeos_with_codecs_release_bot',
+      'Linux ChromeOS Buildspec Tests': 'chromeos_with_codecs_release_bot',
     },
 
     # Take care when changing any of these builders to ensure that you do not