Reduce concurent context counts in offlineaudio-thread-smoke-test.html

1000 contexts were causing time out in Linux and Mac OS bots. Lowering
the count to 500.

Bug: 1092145
Change-Id: Iaee7b4b408e551236252b3b2f163240249c5697b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309805Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790986}

third_party/blink/web_tests/webaudio/OfflineAudioContext/offlineaudiocontext-thread-smoke-test.html

@@ -19,10 +19,9 @@
       const numberOfChannels = 1;
       const renderLength = 1;
 
-      // Create 1000 contexts in total. 58.0.3029 on OSX crashes around 2030,
-      // but creating ~2000 threads makes some try bots time-out.
-      // (crbug.com/741699)
-      const maxNumberOfContexts = 1000;
+      // 1000 concurrent contexts caused this test to time out on Mac OS and
+      // Linux. (See: crbug.com/1092145, crbug.com/741699)
+      const maxNumberOfContexts = 500;
 
       let contexts = [];