Initialize render_frame_host_ in the constuctor of ServiceWorkerHandler.

Initialize render_frame_host_ in the constuctor of ServiceWorkerHandler.

The crash (crbug.com/508775) in ServiceWorkerHandler::UpdateHosts() seems to be caused by wrong state of render_frame_host_.

After ServiceWorkerHandler is created in the constructor of RenderFrameDevToolsAgentHost, ServiceWorkerHandler::SetRenderFrameHost() must be called before UpdateHosts() is called.
So render_frame_host_ must be set correctly before UpdateHosts() is called even if we are not initializing render_frame_host_ now.
So I think this change doesn't fix the crash.
But to make the code safe, we should initialize render_frame_host_ in the constructor.

BUG=508775
Committed: https://crrev.com/149f2315e7d239f66ab9e8635117b099c6976685
Cr-Commit-Position: refs/heads/master@{#338290}

[horo, 2015-07-10 11:38:47]

horo@chromium.org changed reviewers:
+ pfeldman@chromium.org

[horo, 2015-07-10 11:38:48]

pfeldman@
Could you please review this?

[dgozman, 2015-07-10 12:22:13]

dgozman@chromium.org changed reviewers:
+ dgozman@chromium.org

[dgozman, 2015-07-10 12:22:13]

lgtm

[horo, 2015-07-10 14:11:37]

The CQ bit was checked by horo@chromium.org

[commit-bot: I haz the power, 2015-07-10 14:11:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1233633002/1

[commit-bot: I haz the power, 2015-07-10 15:02:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-10 15:03:54]

Patchset 1 (id:??) landed as
https://crrev.com/149f2315e7d239f66ab9e8635117b099c6976685
Cr-Commit-Position: refs/heads/master@{#338290}