report correct content size in AtomNSWindow

The views framework relies on NSWindow to return content size of window,
since we don't use the borderless window, the original result would
include titlebar. We have to override the function to return correct
result for frameless window.

atom/browser/ui/cocoa/atom_ns_window.h

@@ -37,8 +37,10 @@ class ScopedDisableResize {
 @property BOOL disableKeyOrMainWindow;
 @property NSPoint windowButtonsOffset;
 @property(nonatomic, retain) NSView* vibrantView;
-- (void)setShell:(atom::NativeWindowMac*)shell;
+- (id)initWithShell:(atom::NativeWindowMac*)shell
+          styleMask:(NSUInteger)styleMask;
 - (atom::NativeWindowMac*)shell;
+- (NSRect)originalContentRectForFrameRect:(NSRect)frameRect;
 - (void)enableWindowButtonsOffset;
 - (void)toggleFullScreenMode:(id)sender;
 @end