Remove default branch from enum-based switch.

WebCore::ResourceFetcher::determineTargetType has a switch on
Resource::Type, but uses a default branch. This removes the default
branch, so the compiler can warn about missing enum values.

BUG=50132

Review URL: https://codereview.chromium.org/47743002

git-svn-id: svn://svn.chromium.org/blink/trunk@160729 bbb929c8-8fbe-4397-9dbb-9b2b20218538