mac: Let task manager use IDR_PRODUCT_LOGO_16 instead of NSApplicationIcon.

According to `purge; sample -wait Chromium` and launching
out/Release/Chromium.app/Contents/MacOS with 30 wikipedia tabs, this reduces
time spent creating the TaskManagerModel (which happens on the IO thread, in
response to raw bytes arriving from the network) from ~100 samples to ~1 sample.
The icon looks slightly different from the icon that Activity Monitor displays,
but a) it looks arguably nicer now (sharp) and b) most people never open the
task manager.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/190533009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255797 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255797