Drop the 'WebURLRequest::TargetType' enum.

We no longer use this enum in Chromium; everything has been ported over
to using '::RequestContext' and '::FrameType', so we can safely remove
both the enum and the translation machinery in '::setTargetType'.

BUG=390497

Review URL: https://codereview.chromium.org/417483002

git-svn-id: svn://svn.chromium.org/blink/trunk@179125 bbb929c8-8fbe-4397-9dbb-9b2b20218538