WebView: Log field trials when a dummy app is present

Create an empty "WebView Log Verbosifier" app. When installed, WebView
will log all registered field trials and their assigned groups, for QA
purposes. Check for the dummy app asynchronously, because PackageManager
is slow.

Skipping presubmit for LOG(INFO).

BUG=892870
NOPRESUBMIT=true

Change-Id: Ia4fee870aec84a60e9e70642405c5ff8b07da7cf
Reviewed-on: https://chromium-review.googlesource.com/c/1266611
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603996}

• BUILD.gn[diff]
• android_webview/glue/java/src/com/android/webview/chromium/WebViewChromiumAwInit.java[diff]
• android_webview/tools/webview_log_verbosifier/AndroidManifest.xml[Added - diff]
• android_webview/tools/webview_log_verbosifier/BUILD.gn[Added - diff]
• android_webview/tools/webview_log_verbosifier/README[Added - diff]
• base/android/field_trial_list.cc[diff]
• base/android/java/src/org/chromium/base/FieldTrialList.java[diff]