Presubmit change to add android trybots when needed.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index aa79141a90fa5217e6f29f1b7646c0744a21ffd5..fd1df328f8825376e7c0fd28236bcaa54559f58c 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -342,4 +342,17 @@ def GetPreferredTrySlaves(project, change):
   aura_re = '_aura[^/]*[.][^/]*'
   if any(re.search(aura_re, f.LocalPath()) for f in change.AffectedFiles()):
     preferred.append('linux_chromeos')
+  # For bringup (staging of upstream work) we must be careful to not
+  # overload Android infrastructure.  Keeping Android try decisions in a
+  # single location (instead of adding conditionals in base/, net/, ...)
+  # will help us avoid doing so.  For example, we are starting off with
+  # 2 trybots (compared against ~45 for Mac and Linux).
+  # If any file matches something compiled on the main waterfall
+  # android builder, use the android try server.
+  android_re_list = ('^base/', '^ipc/', '^net/', '^sql/', '^jingle/',
+                     '^build/common.gypi$')
+  for f in change.AffectedFiles():
+    if any(re.search(r, f.LocalPath()) for r in android_re_list):
+      preferred.append('android')
+      break
   return preferred