Merge 131673 - Make WindowImpl take into account the icon in the class registrar.

Merge 131673 - Make WindowImpl take into account the icon in the class registrar.

This fixes the bug where an incorrect icon was being
shown in the Windows Task Manager and Task Switcher.

The problem was that the ClassRegistrar was re-using
window classes between different windows, but its
lookup function was not taking into account the icon.

This caused the main Chrome window to take on the
class registered by SingletonHwnd, which did not have
an icon set.

BUG=118368, 117687
TEST=Launch Chrome and check that Chrome's icon
looks correct in the Task Manager and Task Switcher.

Review URL: http://codereview.chromium.org/10031045

TBR=asvitkine@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=131756