Android: moving Webview seed fetch to thread pool

Currently, AsyncTask.execute() defaults to the SERIAL_EXECUTOR. This
exector is good for preventing concurrency errors since it guarantees
serial execution, but bad for performance since the entire app shares
this single queue.

It looks like this callsite can use the THREAD_POOL_EXECUTOR instead,
since this use doesn't appear to rely on the concurrency guarantees that
SERIAL_EXECUTOR provides.

Bug: 869907
Change-Id: I7dce5b4f28520a3d2a81491756f5aec5f6806fa9
Reviewed-on: https://chromium-review.googlesource.com/1158962Reviewed-by: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579862}

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

@@ -163,7 +163,7 @@ public class AwVariationsSeedFetcher extends JobService {
         // If this process has survived since the last run of this job, mFetchTask could still
         // exist. Either way, (re)create it with the new params.
         mFetchTask = new FetchTask(params);
-        mFetchTask.execute();
+        mFetchTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
         return true;
     }