Introduce background Download process to android

chrome/android/java/AndroidManifest.xml

Index: chrome/android/java/AndroidManifest.xml
diff --git a/chrome/android/java/AndroidManifest.xml b/chrome/android/java/AndroidManifest.xml
index b21f383843ea300c1ff7495f9142b1e2034eedb8..b39b3005dea431e6249cc79fd19c0187551a87bd 100644
--- a/chrome/android/java/AndroidManifest.xml
+++ b/chrome/android/java/AndroidManifest.xml
@@ -642,6 +642,13 @@ android:value="true" />
             android:exported="false" />
         {% endfor %}
 
+        <!-- Background download process, it may outlive chrome if download does not finish
+             when user kills chrome -->

[asanka, 2016/01/25 16:44:50]

Is this comment accurate? And if so, was there consensus that the download
process would outlive the browser process in the case where the user explicitly
kills the app (as opposed to Chrome being killed due to other reasons)?

I'm okay if that was the decision, but I want to make sure we are not violating
platform norms or user expectations.

[qinmin, 2016/01/25 22:48:23]

I think this should be the use case and should be expected by user, we can
confirm it with Sidv and Grace. 
The current behavior with Android DownloadManager is that when user explicitly
killing chrome, the download will continue. And we should match that behavior.

+        <service android:name="org.chromium.content.app.DownloadProcessService"
+            android:exported="false"
+            android:permission="{{ manifest_package }}.permission.CHILD_SERVICE"

[asanka, 2016/01/25 16:44:50]

Should this be left unset? What protections do we get from setting this to
CHILD_SERVICE?

Note that we want to restrict all other processes other than the browser process
from binding to this.

[qinmin, 2016/01/25 22:48:23]

This specifies that only processes with CHILD_SERVICE permission can start this
service, which means other apps cannot launch this. Since the GPU and renderer
process will or is already isolated process, they don't have any permissions,
and will not be able to bind or start this service

+            android:process=":download"/>

[svaldez, 2016/01/25 15:33:43]

nit: Reorder to match ordering of previous service definitions:

service android:name=... android:process=... android permission=...
android:isolatedProcess=... android:exported=...

[qinmin, 2016/01/25 22:48:23]

Done.

+
         <receiver android:name="org.chromium.chrome.browser.download.DownloadBroadcastReceiver"
             android:exported="false">
             <intent-filter>