Revert "webview: don't use UI thread pool unnecessarily."

This reverts commit e44d3738.

Reason for revert: It looks to have been necessary after all; crbug.com/1107830

Original change's description:
> webview: don't use UI thread pool unnecessarily.
> 
> Just use the general pool at best-effort priority for logging command
> line and field trials during startup, since it doesn't need to be on the
> UI thread.
> 
> Change-Id: I73dfb14ae67bcc953fa7c90b5840354d5b4a84b3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302946
> Reviewed-by: Nate Fischer <ntfschr@chromium.org>
> Commit-Queue: Richard Coles <torne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#789538}

TBR=torne@chromium.org,ntfschr@chromium.org,hazems@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Fixed: 1107830
Change-Id: If9f8038787cc0a16377aca599cbcb18efa055463
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316414Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791897}

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromiumAwInit.java

@@ -51,7 +51,7 @@ import org.chromium.base.library_loader.LibraryLoader;
 import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.base.metrics.ScopedSysTraceEvent;
 import org.chromium.base.task.PostTask;
-import org.chromium.base.task.TaskTraits;
+import org.chromium.content_public.browser.UiThreadTaskTraits;
 import org.chromium.net.NetworkChangeNotifier;
 import org.chromium.ui.base.ResourceBundle;
 
@@ -457,7 +457,7 @@ public class WebViewChromiumAwInit {
     // Log extra information, for debugging purposes. Do the work asynchronously to avoid blocking
     // startup.
     private static void logCommandLineAndActiveTrials() {
-        PostTask.postTask(TaskTraits.BEST_EFFORT, () -> {
+        PostTask.postTask(UiThreadTaskTraits.BEST_EFFORT, () -> {
             // TODO(ntfschr): CommandLine can change at any time. For simplicity, only log it
             // once during startup.
             AwContentsStatics.logCommandLineForDebugging();