[Sync] Add support for retrying a getupdates due to a context change

sync/engine/backoff_delay_provider_unittest.cc

Index: sync/engine/backoff_delay_provider_unittest.cc
diff --git a/sync/engine/backoff_delay_provider_unittest.cc b/sync/engine/backoff_delay_provider_unittest.cc
index 1bf17cbd5f5fc10c759c24148c9cd36ad4025870..2e5818665f2f94b092e2e0f1ae00323078e7dfe9 100644
--- a/sync/engine/backoff_delay_provider_unittest.cc
+++ b/sync/engine/backoff_delay_provider_unittest.cc
@@ -57,6 +57,10 @@ TEST_F(BackoffDelayProviderTest, GetInitialDelay) {
   EXPECT_EQ(kInitialBackoffRetrySeconds,
             delay->GetInitialDelay(state).InSeconds());
 
+  state.last_download_updates_result = DATATYPE_TRIGGERED_RETRY;
+  EXPECT_EQ(kInitialBackoffImmediateRetrySeconds,
+            delay->GetInitialDelay(state).InSeconds());
+
   state.last_download_updates_result = SYNCER_OK;
   // Note that updating credentials triggers a canary job, trumping
   // the initial delay, but in theory we still expect this function to treat
@@ -99,6 +103,10 @@ TEST_F(BackoffDelayProviderTest, GetInitialDelayWithOverride) {
   EXPECT_EQ(kInitialBackoffShortRetrySeconds,
             delay->GetInitialDelay(state).InSeconds());
 
+  state.last_download_updates_result = DATATYPE_TRIGGERED_RETRY;
+  EXPECT_EQ(kInitialBackoffImmediateRetrySeconds,
+            delay->GetInitialDelay(state).InSeconds());
+
   state.last_download_updates_result = SYNCER_OK;
   // Note that updating credentials triggers a canary job, trumping
   // the initial delay, but in theory we still expect this function to treat