[Android WebView] Upstream WebViewShell to chromium.

WebViewShell used to live in the android repository. This patch
upstreams it to the chromium repository and adds a new target
android_webview_shell_apk. That way both the AndroidWebViewShell.apk
and SystemWebViewGoogle.apk can be built at the same time using one
checkout.

In particular this patch does the following:
- copy the relevant files residing in the android repository under
  frameworks/webview/chromium/tools/WebViewShell/
- add necessary gyp files
- rename the package name to org.chromium.webview_shell
- some import clean ups and formatting
- replace license headers

BUG=475084

Review URL: https://codereview.chromium.org/1070563005

Cr-Original-Commit-Position: refs/heads/master@{#325854}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 86221dd2181ee4e29a0c30a8674ae263007e165f

all.gyp

@@ -85,6 +85,7 @@
               'dependencies': [
                 '../android_webview/android_webview.gyp:android_webview_apk',
                 '../android_webview/android_webview.gyp:system_webview_apk',
+                '../android_webview/android_webview_shell.gyp:android_webview_shell_apk',
                 '../android_webview/android_webview_telemetry_shell.gyp:android_webview_telemetry_shell_apk',
                 '../chrome/chrome.gyp:chrome_shell_apk',
                 '../chrome/chrome.gyp:chrome_sync_shell_apk',