Since WebApkInstaller already knows when the installation is complete or got canceled, we can delete the downloaded apk then.

Note to self: We need to handle the case of the user terminating Chrome prior to the WebAPK getting installed. I do not think that WebApkInstaller gets notified if the install gets cancelled due to Chrome being terminated

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3f0eca90e1ae6168837a85f01a644b729d326f67

commit 3f0eca90e1ae6168837a85f01a644b729d326f67
Author: hanxi <hanxi@chromium.org>
Date: Wed Sep 28 03:49:25 2016

Download WebAPK into WebAPK-specifc directory, and delete all cached
APKs before downloading a new one.

BUG= 634208 

Review-Url: https://codereview.chromium.org/2364133002
Cr-Commit-Position: refs/heads/master@{#421439}

[modify] https://crrev.com/3f0eca90e1ae6168837a85f01a644b729d326f67/chrome/browser/android/webapk/webapk_installer.cc
[modify] https://crrev.com/3f0eca90e1ae6168837a85f01a644b729d326f67/chrome/browser/android/webapk/webapk_installer.h