Remove tests and code for showModalDialog

This deletes the chromium-side implementation logic and a few tests for
showModalDialog. I believe more simplification is possible to do without the
design constraints imposed by showModalDialog but we should probably start
small in case we run into unexpected compat issues.

R=darin
BUG=345831

Review URL: https://codereview.chromium.org/165793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252883 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252883