Apply scaleTimeout to the default timeout in CallbackHelper

If we wanted to support fractional scales well we'd need to switch to
using some smaller unit, such as milliseconds, but that doesn't seem to
be the purpose of scaleTimeout (the example given in the documentation
is scale == 20), and it would require touching many files.

Bug: none
Change-Id: I97b625d10b3be5ddbd305694ab78714a43677f08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084717Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746638}

base/test/android/javatests/src/org/chromium/base/test/util/CallbackHelper.java

@@ -126,7 +126,7 @@ import java.util.concurrent.TimeoutException;
  */
 public class CallbackHelper {
     /** The default timeout (in seconds) for a callback to wait. */
-    public static final long WAIT_TIMEOUT_SECONDS = 5L;
+    public static final long WAIT_TIMEOUT_SECONDS = ScalableTimeout.scaleTimeout(5L);
 
     private final Object mLock = new Object();
     private int mCallCount;