Use ApplicationContext instead of Activity

Use ApplicationContext instead of Activity

In document mode, multiple activities could be used concurrently.
So registering and unregistering a receiver could happen on different ones,
which causes Chrome crash.

BUG=530097
Committed: https://crrev.com/a774b242861f73871f663c8bf4030908684fd6bf
Cr-Commit-Position: refs/heads/master@{#348741}

[Jaekyun Seok (inactive), 2015-09-14 01:25:11]

jaekyun@chromium.org changed reviewers:
+ aurimas@chromium.org, tedchoc@chromium.org

[Jaekyun Seok (inactive), 2015-09-14 01:25:12]

aurimas@ and tedchoc@, please review this CL.

[Ted C, 2015-09-14 15:59:18]

lgtm

[Jaekyun Seok (inactive), 2015-09-14 22:11:57]

The CQ bit was checked by jaekyun@chromium.org

[commit-bot: I haz the power, 2015-09-14 22:13:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1338813004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1338813004/1

[commit-bot: I haz the power, 2015-09-14 22:20:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-14 22:20:53]

Patchset 1 (id:??) landed as
https://crrev.com/a774b242861f73871f663c8bf4030908684fd6bf
Cr-Commit-Position: refs/heads/master@{#348741}

[commit-bot: I haz the power, 2015-09-23 12:38:07]

Patchset 1 (id:??) landed as
https://crrev.com/a774b242861f73871f663c8bf4030908684fd6bf
Cr-Commit-Position: refs/heads/master@{#348741}