Revert of Change HungPluginTabHelper to listen for infobar changes through Observer style.

Revert of Change HungPluginTabHelper to listen for infobar changes through Observer style. (https://codereview.chromium.org/297293002/)

Reason for revert:
Caused a crash on Windows. See crbug.com/354380

Original issue's description:
> Change HungPluginTabHelper to listen for infobar changes through Observer style.
> 
> Test plan:
> 
> 1) out/Debug/chrome --no-sandbox --ppapi-flash-path=/opt/google/chrome/PepperFlash/libpepflashplayer.so --ppapi-flash-version=`grep -i version /opt/google/chrome/PepperFlash/manifest.json | awk '{print $2}' | awk -F"\"" '{print $2}'`
> 2) Go to a site that has flash, e.g., http://www.flash-game.net/
> 3) Open Task Manager, Shift+Esc
> 4) Search for "Plug-in: Shockwave Flash" in the list.
> 5) Select it and click on "End process" button.
> 6) Go to the website and observe the infobar.
> 
> Another way to test it is:
> 
> 1) Add an infinite loop into ppapi/examples/scripting/post_message.cc
> 2) Build ppapi_example_post_message
> 3) out/Debug/chrome --no-sandbox --register-pepper-plugins="/home/tfarina/chromium/src/out/Debug/lib/libppapi_example_post_message.so;application/x-ppapi-post-message-example" ppapi/examples/scripting/post_message.html
> 
> BUG=354380
> TEST=see above
> R=pkasting@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277292
NOTRY=true
TBR=pkasting@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277804

[tfarina, 2014-06-17 17:50:03]

Created Revert of Change HungPluginTabHelper to listen for infobar changes
through Observer style.

[commit-bot: I haz the power, 2014-06-17 17:52:28]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tfarina@chromium.org/338353003/1

[commit-bot: I haz the power, 2014-06-17 17:52:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-17 17:52:30]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Peter Kasting, 2014-06-17 17:56:17]

LGTM

[Peter Kasting, 2014-06-17 17:56:24]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2014-06-17 17:58:22]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tfarina@chromium.org/338353003/1

[commit-bot: I haz the power, 2014-06-17 18:03:53]

Change committed as 277804