Make ClearBrowsingDataHandler only observe its own removal task

Make ClearBrowsingDataHandler only observe its own removal task

In https://codereview.chromium.org/2133893002/, ClearBrowsingDataHandler
was made to observe whether BrowsingDataRemover has any removal task
in progress and disable the "Clear data" button if that is the case.

This is no longer necessary. BrowsingDataRemover is nowadays able to
accept a new removal task at any time and call back (only) the associated
observer when the task is done. Callers no longer need to be aware of
the possibility of other removal tasks running.

We still disable the "Clear data" button while we execute the removal
task started by pressing that button - this serves as a feedback to the
user that their removal task has indeed been executed, i.e. that pressing
the button had an effect. However, opening another instance of a dialog
in a different tab (i.e. tied to a different handler) allows the user
to execute another parallel removal task if they wish to do so.

Finally, we remove the OnBrowsingDataRemoverRemoving(bool) method from
BrowsingDataRemover::Observer, as the previous behavior of
ClearBrowsingDataHandler was its only usecase.

In practice, this CL is mostly a partial revert of
https://codereview.chromium.org/2133893002/

BUG=630327
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2240883002
Cr-Commit-Position: refs/heads/master@{#415925}
(cherry picked from commit a946e92eaa55cb6da801ada49dfdd51d786fa06d)

[msramek, 2016-09-13 15:02:28]

This was merged in
https://bugs.chromium.org/p/chromium/issues/detail?id=630327#c9, but the review
wasn't closed or commented on for some reason. Closing it.