Remove an unneccessary special case for the main resource from InspectorPageClient

Miniature cleanup, the special case is not needed as the default: case handles
main resources well: it returns early with false as resourceBuffer() is null.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/350263002

git-svn-id: svn://svn.chromium.org/blink/trunk@177032 bbb929c8-8fbe-4397-9dbb-9b2b20218538