Add test case for Flutter Issue #27677 as a benchmark. (#34870)

* Add test case for Flutter Issue #27677 as a benchmark.

See https://github.com/flutter/flutter/issues/27677

I got the following results running the test on a Moto E2 which
will help us determine how much we can gain by analyzing the
operations and eliminating unnecessary repaints based on
dirty rectangles/regions.

no blurs       - avg 216.0 fps over 3 samples
blur the group - avg  22.9 fps over 3 samples
blur each txt  - avg   3.4 fps over 3 samples

* Added the new benchmark to be tracked on the dashboard.

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index f142262..5ecb300 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -171,6 +171,13 @@
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
 
+  backdrop_filter_perf__timeline_summary:
+    description: >
+      Measures the runtime performance of backdrop filter blurs on Android.
+    stage: devicelab
+    required_agent_capabilities: ["mac/android"]
+    flaky: true
+
   flavors_test:
     description: >
       Checks that flavored builds work on Android.