clang-format: Improve formatting of ObjC method expressions.

In particular, make breaking after a parameter's ":" more of a last
resort choice as it significantly affects the readability gained by
aligning the parameters.

Before (in Chromium style - which doesn't allow bin-packing):
  {
    popup_window_.reset([[RenderWidgetPopupWindow alloc]
        initWithContentRect:
            NSMakeRect(
                origin_global.x, origin_global.y, pos.width(), pos.height())
                  styleMask:NSBorderlessWindowMask
                    backing:NSBackingStoreBuffered
                      defer:NO]);
  }

After:
  {
    popup_window_.reset([[RenderWidgetPopupWindow alloc]
        initWithContentRect:NSMakeRect(origin_global.x,
                                       origin_global.y,
                                       pos.width(),
                                       pos.height())
                  styleMask:NSBorderlessWindowMask
                    backing:NSBackingStoreBuffered
                      defer:NO]);
  }

llvm-svn: 195301