Change PRESUBMIT to try more cls on android

- android doesn't currently build anything in chrome/
  so we're changing try job logic to send builds to android
  unless the CL was exclusively in chrome/

Previously we would have built on roughly 20% of cls with 'git try'
After this change we will build on roughly 60% of cls (based on last 10k
commits)

BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10694029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@144841 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 2c07248..aaf6544 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -480,6 +480,9 @@
 def GetPreferredTrySlaves(project, change):
   files = change.LocalPaths()
 
+  if not files:
+    return []
+
   if all(re.search('\.(m|mm)$|[/_]mac[/_.]', f) for f in files):
     return ['mac_rel']
   if all(re.search('[/_]win[/_.]', f) for f in files):
@@ -492,9 +495,7 @@
   if any(re.search('[/_]aura', f) for f in files):
     trybots.append('linux_chromeos')
 
-  # Ensure CL contains some relevant files (i.e. not purely ^chrome)
-  if any(re.search('^(base|build|content|ipc|jingle|media|net|sql)/', f)
-         for f in files):
+  if not all(f.startswith('chrome/') for f in files):
     trybots.append('android')
 
   return trybots