Add basic instructions on how to use CCT with Chromium

BUG=none

Change-Id: I0c207a6cb2131ede179b5c03aed4cb58277363b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606594Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659610}

third_party/custom_tabs_client/testing_with_chromium.md

+# Testing Chrome Custom Tabs with Chromium
+
+[TOC]
+
+Chrome Custom Tabs (CCT) allow an app to customize how Chrome looks and feels.
+It gives an app more control over the web experience without having to use
+WebView.
+
+## What is Chrome Custom Tabs?
+
+For more information on Chrome Custom Tabs, refer to
+[this blog post](https://developer.chrome.com/multidevice/android/customtabs).
+If you want to use Chrome Custom Tabs in your own app, the instructions are
+[here](/third_party/custom_tabs_client/src/Using.md).
+
+## Building
+
+These instruction assume that you have already built Chromium for Android. If
+not, instructions for building Chromium for Android are
+[here](/docs/android_build_instructions.md). Details below assume that the
+build is setup in out/Default.
+
+### Build Chrome Custom Tabs example
+
+```shell
+$ autoninja -C out/Default custom_tabs_client_example_apk
+```
+
+### Install Chrome Custom Tabs example
+
+```shell
+# Install the example
+$ out/Default/bin/custom_tabs_client_example_apk install
+```
+
+## Start running the app
+
+**That it!** The example app should be installed and available. Once you
+launch "Chrome Custom Tabs Example" ("Chrome C..." in apps), you should be
+able to switch to use Chromium by changing "Package" to Chromium (or any
+version of Chrome installed on the device). Then simply click "Launch URL
+in a Chrome Custom Tab"