perf: Make RAF heavy first frame test do what it says

The requestAnimationFrame test which simulates a large jank in the
first rAF callback (raf.html?heavy_first_frame) was actually adding
the delay to every callback. This patch fixes the synthetic delay
to use the correct one-shot mode.

BUG=391005

Review URL: https://codereview.chromium.org/497173002

Cr-Commit-Position: refs/heads/master@{#291886}

tools/perf/page_sets/tough_scheduling_cases.py

@@ -249,7 +249,8 @@ class Page16(ToughSchedulingCasesPage):
       url='file://tough_scheduling_cases/raf.html?heavy_first_frame',
       page_set=page_set)
 
-    self.synthetic_delays = {'cc.BeginMainFrame': {'target_duration': 0.15}}
+    self.synthetic_delays = {'cc.BeginMainFrame': {'target_duration': 0.15,
+                                                   'mode': 'oneshot'}}
 
 
 class Page17(ToughSchedulingCasesPage):