Schedule network3g tests only on branches >= R37.

Due to chromium:389628, the cellular DUTs are being held up by
network_3GDisableWhileConnecting for a long period of time. The issue
has been fixed in R37 and ToT, which still affects R36. As a workaround,
we will only schedule network3g tests on branches >= R37.

BUG=chromium:389628
TEST=site_utils/suite_scheduler/suite_scheduler.py --sanity

Change-Id: I3cece9dd5f4b32c1e7c63a30eed741aef199a1e1
Reviewed-on: https://chromium-review.googlesource.com/210077
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/suite_scheduler.ini b/suite_scheduler.ini
index b278821..21d298e 100644
--- a/suite_scheduler.ini
+++ b/suite_scheduler.ini
@@ -136,27 +136,27 @@
 [Network3g]
 run_on: nightly
 suite: network3g
-branch_specs: >=R25
+branch_specs: >=R37
 pool: cellular
 num: 7
 
 [Network3g_att]
 run_on: nightly
 suite: network3g_att
-branch_specs: >=R36
+branch_specs: >=R37
 pool: cellular
 
 [Network3g_tmobile]
 run_on: nightly
 suite: network3g_tmobile
-branch_specs: >=R28
+branch_specs: >=R37
 pool: cellular
 num: 4
 
 [Network3g_verizon]
 run_on: nightly
 suite: network3g_verizon
-branch_specs: >=R28
+branch_specs: >=R37
 pool: cellular
 num: 2