Revert 212528 "Cleanup: remove redundant tab close observation f..."

First of two-part revert of r212329 due to print preview dialog
breakage. r212528 builds on top of r212329 and must be reverted first.

> Cleanup: remove redundant tab close observation from TabModalConfirmDialogDelegate 
> 
> The WebContentsModalDialogManager closes web contents modal dialogs 
> when their corresponding WebContents is destroyed. This supersedes the 
> TabModalConfirmDialogDelegate handling of the tab close event. 
> 
> BUG=157161
> 
> Review URL: https://chromiumcodereview.appspot.com/18786005

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Review URL: https://codereview.chromium.org/21113005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214449 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed