Partially deprecate the old webstorePrivate.beginInstall method

Partially deprecate the old webstorePrivate.beginInstall method

We are still allowing it to be used to do installs, but with this change it
will no longer prevent the confirmation dialog from appearing. Once the web
store completely transitions to use the newer beginInstallWithManifest2
method, we can remove beginInstall alltogether.

BUG=75821
TEST=Installing an app/extension from the web store should cause an install 
confirmation prompt to appear after the item downloads.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=88204

[Matt Perry, 2011-06-07 18:03:30]

LGTM except for below

http://codereview.chromium.org/7131001/diff/1/chrome/browser/extensions/exten...
File chrome/browser/extensions/extension_gallery_install_apitest.cc (right):

http://codereview.chromium.org/7131001/diff/1/chrome/browser/extensions/exten...
chrome/browser/extensions/extension_gallery_install_apitest.cc:55: virtual void
Observe(NotificationType type,
Don't you need to call the parent class' Observe? Also, you're not guaranteed to
be getting the EXTENSION_WILL_SHOW_CONFIRM_DIALOG notification, since the base
class could subscribe to some.

[asargent_no_longer_on_chrome, 2011-06-07 18:40:32]

In order to avoid conflicts with the observe of the parent class, I moved the
handling of the EXTENSION_WILL_SHOW_CONFIRM_DIALOG event into a helper method. 

Please take another look.

[Matt Perry, 2011-06-07 18:54:18]

LGTM