This method returns contextDebugId for function. We can't use context_data from FunctionMirror.prototype.script because it can be incorrect when compilation cache is used and one script object was used for JSFunctions in different contexts.

BUG=chromium:595206
LOG=Y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1840713002

Cr-Commit-Position: refs/heads/master@{#35117}