Add a metric to count the checked data types during password deletion.

Add a metric to count the checked data types during password deletion.

The deletion of passwords is a very destructive operation; we presume that this only happens if the user wipes all their browsing data, i.e. all checkboxes in the ClearBrowsingData dialog are checked. Other usecases, such as deleting only the passwords, should be less common. Various subsets of selected data types should be even less common.

The newly added metric tests this assumption by counting how many data types are checked when the passwords are checked in the Clear Browsing Data dialog.

BUG=529054

Review URL: https://codereview.chromium.org/1313513010

Cr-Commit-Position: refs/heads/master@{#348606}