[Mac System Hotkeys] Skip undocumented AppleSymbolicHotKeys

When you change the shortcuts for Spaces Left and Spaces Right, macOS
inserts your new shortcuts at slots 79 and 81 in AppleSymbolicHotKeys.
For some reason, it also inserts "shifted" versions of these
shortcuts at slots 80 and 82. It's unclear why these shifted versions
exist because the shortcuts don't seem to do anything. They could be
entries macOS adds for some reason but knows to skip (not expecting
any non-system application to read this file). Treating these
shortcuts as valid creates unexpected behavior. For example, in the
case of "Spaces Left" being mapped to Option Left Arrow, this extra
shortcut causes Chrome to silently ignore Shift-Option Left Arrow,
the shortcut which appends the current text selection by one word to
the left. This cl works around the problem by changing the
SystemHotkeyMap to ignore these two undocumented shortcuts.

This cl also forces an eventual reload of the SystemHotkeyMap after
the user switches away from Chrome and back, to ensure any new
shortcut settings are recognized.

Bug: 874219

Test: out/Default/content_unittests \
    --gtest_filter=SystemHotkeyMapTest.IgnoreUndocumentedShortcutEntries

Change-Id: Ifc17ec48877de8aeae01244794ec5fcf2444fe5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3908188
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jayson Adams <shrike@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1077678}