Presubmit change to add android trybots when needed.

Also add warning in android_all.gyp that extra targets in 'all' can
have a significant impact on the try load so coordinate appropriately.

BUG=None
TEST=


Review URL: http://codereview.chromium.org/9360028

git-svn-id: http://src.chromium.org/svn/trunk/src/build@121560 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

all_android.gyp

@@ -21,6 +21,10 @@
       # until the full set supported.  If adding a new test here,
       # please also add it to build/android/run_tests.py, else the
       # test is not run.
+      #
+      # WARNING:
+      # Do not add targets here without communicating the implications
+      # on tryserver triggers and load.  Discuss with jrg please.
       'target_name': 'android_builder_tests',
       'type': 'none',
       'dependencies': [