Add TargetIsPing and have PingLoader use it.

We'll use it to detach requests from renderers. This is used both for
PingLoader::loadImage and PingLoader::sendPing. Temporarily define a
WEBKIT_HAS_TARGET_IS_PING preprocessor symbol until Chromium downstream
is updated.

(Even though the former comes from loading an image, it's part of the
special-case behavior where images loaded in unload handlers don't get
cancelled, and doesn't load like a normal image, so we shouldn't be treating it
as one.)

BUG=302816

Review URL: https://codereview.chromium.org/55203003

git-svn-id: svn://svn.chromium.org/blink/trunk@162565 bbb929c8-8fbe-4397-9dbb-9b2b20218538