Disable BackgroundRenderersAreThrottled on all platform

Flakiness dashboard shows that it has been flaky for a while

TBR=wjmaclean@chromium.org
NOTRY=true

Bug: 874497
Change-Id: I017096a3a33ba69cab80cf9f78caa59bd4672319
Reviewed-on: https://chromium-review.googlesource.com/1175939Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583289}

third_party/blink/renderer/core/scheduler/throttling_test.cc

@@ -59,7 +59,8 @@ TEST_F(DisableBackgroundThrottlingIsRespectedTest,
 
 class BackgroundRendererThrottlingTest : public SimTest {};
 
-TEST_F(BackgroundRendererThrottlingTest, BackgroundRenderersAreThrottled) {
+TEST_F(BackgroundRendererThrottlingTest,
+       DISABLED_BackgroundRenderersAreThrottled) {
   SimRequest main_resource("https://example.com/", "text/html");
 
   LoadURL("https://example.com/");