https://codereview.chromium.org/1781433002 enabled auto-resize for
constrained web dialogs on Mac. This updated a number of call sites for "new ConstrainedWindowMac" but the one for sign in was missed: https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm&l=54

This will need to be merged into M50 ASAP.