DevTools: Reset UISourceCode compiler script mapping on debugger reset.

This is needed to allow breakpoint manager understand if UISourceCode is actually mapped to some script.

R=pfeldman

Review URL: https://chromiumcodereview.appspot.com/23653056

git-svn-id: svn://svn.chromium.org/blink/trunk@158178 bbb929c8-8fbe-4397-9dbb-9b2b20218538