Add support for constrained windows on os x, based on Avi's GTMWindowSheetController.

chrome/chrome.gyp

Index: chrome/chrome.gyp
diff --git a/chrome/chrome.gyp b/chrome/chrome.gyp
index eac013437bd282a15158d3ca1ec983154f808e27..5dd047df047415368a22cd1f2c00acf753cc6e7f 100644
--- a/chrome/chrome.gyp
+++ b/chrome/chrome.gyp
@@ -802,6 +802,8 @@
         'browser/cocoa/cocoa_test_helper.h',
         'browser/cocoa/command_observer_bridge.h',
         'browser/cocoa/command_observer_bridge.mm',
+        'browser/cocoa/constrained_window_mac.h',
+        'browser/cocoa/constrained_window_mac.mm',
         'browser/cocoa/custom_home_pages_model.h',
         'browser/cocoa/custom_home_pages_model.mm',
         'browser/cocoa/delayedmenu_button.h',
@@ -812,6 +814,8 @@
         'browser/cocoa/download_item_controller.mm',
         'browser/cocoa/download_item_mac.h',
         'browser/cocoa/download_item_mac.mm',
+        'browser/cocoa/download_request_dialog_delegate_mac.h',
+        'browser/cocoa/download_request_dialog_delegate_mac.mm',
         'browser/cocoa/download_shelf_controller.h',
         'browser/cocoa/download_shelf_controller.mm',
         'browser/cocoa/download_shelf_mac.h',
@@ -2071,6 +2075,7 @@
             '../third_party/GTM/AppKit/GTMUILocalizer.m',
             '../third_party/GTM/AppKit/GTMUILocalizerAndLayoutTweaker.h',
             '../third_party/GTM/AppKit/GTMUILocalizerAndLayoutTweaker.m',
+            '../third_party/GTM/AppKit/GTMWindowSheetController.m',
             # Build necessary Mozilla sources
             '../third_party/mozilla/include/NSPasteboard+Utils.h',
             '../third_party/mozilla/include/NSPasteboard+Utils.mm',