[Extensions] Pass XWalkExtensionFunctionInfo as a scoped_ptr

We were previously passing it as a const reference. Now we are giving
the ownership to the recipient. The next patches will be fixing
the thread safety of this class and the limitation that it cannot
outlive the XWalkExtensionInstance.
[Extensions] Make it safe for XWalkExtensionFunctionInfo to outlive XWalkExtensionInstance

XWalkExtensionFunctionInfo will now send the results to the
XWalkExtensionFunctionHandler, which is a weak reference. Checking if
the weak reference is valid only works on the same thread which
the weak reference was created, but this going to be fixed in the
upcoming patch.
[Extensions] Add more XWalkExtensionFunctionHandler unit tests

This new test is ensuring that calling PostResult in a
XWalkExtensionFunctionInfo after the XWalkExtensionInstance is deleted
doesn't have any bad side effect.
[Extensions] Make the PostResult thread safe

It will now re-post the message to the same thread as the XWalkExtensionFunctionHandler
to certify that there is no race condition when checking if the weak reference
is valid.
Merge pull request #866 from tmpsantos/function_handler

[Extensions] Refactoring of the XWalkExtensionFunctionHandler