customtabs: Fix DetachedResourceRequestTest#testCanDoParallelRequest flakiness.

Bug: 904538
Change-Id: If4099acc9e699c7d6dfc4478b24786cbf931133e
Reviewed-on: https://chromium-review.googlesource.com/c/1335606Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608307}

chrome/android/javatests/src/org/chromium/chrome/browser/customtabs/DetachedResourceRequestTest.java

@@ -77,6 +77,7 @@ public class DetachedResourceRequestTest {
         mContext = InstrumentationRegistry.getInstrumentation()
                            .getTargetContext()
                            .getApplicationContext();
+        ThreadUtils.runOnUiThreadBlocking(OriginVerifier::clearCachedVerificationsForTesting);
     }
 
     @After