These are instructions for building the Android WebView. These instructions are necessary if you wish to run the WebView code as an Android system component (which is useful when working on performance or application compatibility). To run tests and for certain development tasks it is sufficient to use the standalone test shell as described here.
For googlers, see internal instructions.
This workflow currently requires a device with AOSP builds of Android Lollipop 5.0.0 release, with root access. Follow instructions at https://source.android.com/source/building.html to build and flash the correct AOSP version to your device. Here's the rough flow of building AOSP for Nexus 5.
Follow the instructions here for getting the source and building chromium for android.
There is a one time device set up to remove the existing webview. This is needed because it is not possible to install over the existing apk due to mismatched signing key.
Install the built apk.