Use valid sample rate for offline context

The test offlineaudiocontext-detached-execution-context.tentative.html
uses a sample rate of 3000 Hz, but the spec only mandates a minimum
sample rate of 8000 Hz.  Update the test to use a sample rate of 8
kHz.

Bug: 1137014
Test: offlineaudiocontext-detached-execution-context.tentative.html
Change-Id: I7fb2069e8c69f77224aad6f92a8acf9e02cba72d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462233Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816331}

third_party/blink/web_tests/external/wpt/webaudio/the-audio-api/the-offlineaudiocontext-interface/offlineaudiocontext-detached-execution-context.tentative.html

@@ -19,7 +19,7 @@
 
         // Use the lowest value possible for the faster test.
         let context =
-            new iframe.contentWindow.OfflineAudioContext(1, 1, 3000);
+            new iframe.contentWindow.OfflineAudioContext(1, 1, 8000);
 
         document.body.removeChild(iframe);