Following logic is using for getting function name in JSFunction::GetDebugName:
1. if function has displayName and its type is string then use it
2. if function has defined property Function.name as value and its type string then use it
3. otherwise use SharedFunctionInfo::DebugName as functionName.

JSFunction::GetDebugName is exposed in V8 API and in FunctionMirror interface.

BUG=chromium:17356
R=yangguo@chromium.org,mstarzinger@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/1449473005

Cr-Commit-Position: refs/heads/master@{#32124}