[AW] Reduce minimum Variations seed download period to 12 hours.

This CL reduces the minimum amount of time between seed downloads from
24 to 12 hours. This should reduce the time taken for variations seeds
to be distributed on WebView.

12 hours was chosen as a compromise between our need for quick seed
distribution and resource usage on device. We may revisit this number
based on the results of this change.

Test: Manually verified with logs that 12h is used for timestamp check.
Bug: 979075
Change-Id: I5b183f9970662468be1b214cf7d8cb6d4665041d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086263Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748381}

android_webview/nonembedded/java/src/org/chromium/android_webview/services/AwVariationsSeedFetcher.java

@@ -45,7 +45,7 @@ import java.util.concurrent.TimeUnit;
 public class AwVariationsSeedFetcher extends JobService {
     private static final String TAG = "AwVariationsSeedFet-";
     private static final int JOB_ID = TaskIds.WEBVIEW_VARIATIONS_SEED_FETCH_JOB_ID;
-    private static final long MIN_JOB_PERIOD_MILLIS = TimeUnit.DAYS.toMillis(1);
+    private static final long MIN_JOB_PERIOD_MILLIS = TimeUnit.HOURS.toMillis(12);
 
     /** Clock used to fake time in tests. */
     public interface Clock { long currentTimeMillis(); }