Fix startDrag crash on macOS with empty image

[kevinsawicki]

Currently webContents.startDrag will crash on macOS if called with an empty image.

This pull request updates webContents.startDrag to check for a non-empty image on macOS and raise an error when they happen.

Noticed this while trying to reproduce #8445

Crash report:

Crashing on exception: CALayer position contains NaN: [nan nan]

Application Specific Backtrace 1:
0   CoreFoundation                      0x00007fffc2d1de7b __exceptionPreprocess + 171
1   libobjc.A.dylib                     0x00007fffd7907cad objc_exception_throw + 48
2   CoreFoundation                      0x00007fffc2d9c99d +[NSException raise:format:] + 205
3   QuartzCore                          0x00007fffc8ad7f48 _ZN2CA5Layer12set_positionERKNS_4Vec2IdEEb + 152
4   QuartzCore                          0x00007fffc8ad80bd -[CALayer setPosition:] + 44
5   HIToolbox                           0x00007fffc2260f95 _ZL20UpdateLayerPositionsP14OpaqueCoreDrag + 662
6   HIToolbox                           0x00007fffc248f752 _Z29UnflockAnimationTimerCallbackP16__CFRunLoopTimerPv + 784
7   CoreFoundation                      0x00007fffc2c9d244 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
8   CoreFoundation                      0x00007fffc2c9cecf __CFRunLoopDoTimer + 1071
9   CoreFoundation                      0x00007fffc2c9ca2a __CFRunLoopDoTimers + 298
10  CoreFoundation                      0x00007fffc2c943e1 __CFRunLoopRun + 2065
11  CoreFoundation                      0x00007fffc2c93974 CFRunLoopRunSpecific + 420
12  CoreFoundation                      0x00007fffc2cf5da5 CFMessagePortSendRequest + 965
13  HIServices                          0x00007fffc19a0b56 SendDragIPCMessage + 540
14  HIServices                          0x00007fffc199e011 CoreDragMessageHandler + 1321
15  CoreFoundation                      0x00007fffc2d34486 __CFMessagePortPerform + 598
16  CoreFoundation                      0x00007fffc2c9c6c9 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
17  CoreFoundation                      0x00007fffc2c9c641 __CFRunLoopDoSource1 + 465
18  CoreFoundation                      0x00007fffc2c94525 __CFRunLoopRun + 2389
19  CoreFoundation                      0x00007fffc2c93974 CFRunLoopRunSpecific + 420
20  HIToolbox                           0x00007fffc221fa5c RunCurrentEventLoopInMode + 240
21  HIToolbox                           0x00007fffc221f891 ReceiveNextEventCommon + 432
22  HIToolbox                           0x00007fffc221f6c6 _BlockUntilNextEventMatchingListInModeWithFilter + 71
23  AppKit                              0x00007fffc07c55b4 _DPSNextEvent + 1120
24  AppKit                              0x00007fffc0f3fd6b -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 2789
25  AppKit                              0x00007fffc0a66012 NSCoreDragCGEventBlockingProc + 175
26  HIServices                          0x00007fffc1997ccc SampleMouseAndKeyboard + 145
27  HIServices                          0x00007fffc19979b1 DragInApplication + 53
28  HIServices                          0x00007fffc199696c CoreDragStartDragging + 486
29  AppKit                              0x00007fffc0a630cc -[NSCoreDragManager _dragUntilMouseUp:accepted:] + 1048
30  AppKit                              0x00007fffc0a60056 -[NSCoreDragManager dragImage:fromWindow:at:offset:event:pasteboard:source:slideBack:] + 1193
31  AppKit                              0x00007fffc0a5fb9c -[NSWindow(NSDrag) dragImage:at:offset:event:pasteboard:source:slideBack:] + 135
32  Electron Framework                  0x00000001044ba92e _ZN4atom13DragFileItemsERKNSt3__16vectorIN4base8FilePathENS0_9allocatorIS3_EEEERKN3gfx5ImageEP6NSView + 510

Thread 0 Crashed:: CrBrowserMain  Dispatch queue: com.apple.main-thread
0   libbase.dylib                 	0x0000000111474df1 base::debug::BreakDebugger() + 17
1   libbase.dylib                 	0x000000011149792f logging::LogMessage::~LogMessage() + 1391
2   libgfx.dylib                  	0x0000000115f5b957 gfx::Image::ToNSImage() const + 71
3   com.github.electron.framework 	0x000000010ef447a0 atom::DragFileItems(std::__1::vector<base::FilePath, std::__1::allocator<base::FilePath> > const&, gfx::Image const&, NSView*) + 384 (drag_util_mac.mm:49)
4   com.github.electron.framework 	0x000000010edfc6f5 atom::api::WebContents::StartDrag(mate::Dictionary const&, mate::Arguments*) + 2341 (atom_api_web_contents.cc:1338)