Android: move to CVC onShow() / onHide() for child process binding management.

Existing process management logic adjusts child process bindings in response to
ContentViewCore onAttachedToWindow() / onDetachedFromWindow() calls. This
causes some inconveniences: views in the embedding application sometimes are
detached for short-lived animations; but more importantly, we now have
embedders that do not alter the view hierarchy at all, preventing any
management of their renderer bindings

This patch moves to onShow() / onHide() calls instead. These are called by the
embedding applications independently from the view hierarchy changes.

In order to keep the most recently used renderer strongly bound during the
background period (as it was the case under onAttachedToWindow() /
onDetachedFromWindow() calls), explicit logic is introduced in the
BindingManager.

Also, the delay for removing strong bindings is brought from 5s to 1s as we no
longer face the interference of views detaching for short-lived animations (as
it was the case when we were relying on onAttachedToWindow() /
onDetachedFromWindow()).

BUG=317963

Review URL: https://codereview.chromium.org/74943006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237333 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237333