Allow system key equivalents to work again.
Allow system key equivalents to work again.

The changes in http://review.skia.org/300182 caused cmd-Q to stop
working normally in Viewer, because the Mac window would report that it
had processed all key equivalents. This prevented the NSApp from
handling key equivalents in the menu bar. This CL now forwards on all
key equivalents to the system for processing (even as it passes them to
ImGui), allowing cmd-Q to work again.

This CL also simplifies the pattern for updating modifier keys slightly.

Change-Id: I2285839b41dd361e34694eccbc6d581662b24648
Bug: skia:10338
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300651
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Reviewed-by: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia