Better distinguish between blocked and empty pages in XSS filter tests.

We currently detect the XSSAuditor's fully-blocked frames by the inability
of a parent frame to access their contents due to the cross-origin "data:," URL
they are redirected to as a result of the block. An example of this technique
is the "cross origin frame" console message present in many CSP tests.

Before we can improve the mechanism by which fully-blocked frames are handled,
we need to stop relying on this side-effect.  Instead, it is better to dump
all child frames, and see if they have partial contents.

But to do this we must ensure that all tests put some minimal content
into the frame, otherwise a blocked frame and an empty frame produce identical
output.  We do this in the echo-intertag.pl file.

This was split off from a forthcoming CL to reduce the file count in that CL.

Review URL: https://codereview.chromium.org/420603003

git-svn-id: svn://svn.chromium.org/blink/trunk@178939 bbb929c8-8fbe-4397-9dbb-9b2b20218538