[debug] [reland] Consistently use script from FrameMirror

src/inspector/debugger_script_externs.js

Index: src/inspector/debugger_script_externs.js
diff --git a/src/inspector/debugger_script_externs.js b/src/inspector/debugger_script_externs.js
index c7df61f3f49550bbc2087a9658a74e4e3af29661..2a0b92d30ca44547dc44d99ac2d74f4c24d10c87 100644
--- a/src/inspector/debugger_script_externs.js
+++ b/src/inspector/debugger_script_externs.js
@@ -466,6 +466,9 @@ FrameMirror.prototype.allScopes = function(ignoreNestedScopes) {}
 /** @return {!FrameDetails} */
 FrameMirror.prototype.details = function() {}
 
+/** @return {!ScriptMirror} */
+FrameMirror.prototype.script = function() {}
+
 /**
  * @param {string} source
  * @param {boolean} disableBreak