mac: wire up pane traversal keys

This change adds two new global key bindings:
Command-Option-UpArrow traverses to the next accessible pane
Command-Option-Shift-UpArrow traverses to the previous accessible pane

This allows keyboard access to:
* The downloads bar
* Bubbles that don't grab focus when they appear
* The toolbar
etc etc.

This is the approach proposed by UX in
https://bugs.chromium.org/p/chromium/issues/detail?id=919260#c20,
but I decided to use the (imo more Mac-like) "shift means backward"
convention rather than using DownArrow for forward and UpArrow for
backward.

Bug: 919260,956432,945923
Change-Id: I4d15d706b903d1be9d1894725f96107a4b6b1373
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856888
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705285}

chrome/browser/global_keyboard_shortcuts_mac.mm

@@ -168,6 +168,9 @@ const std::vector<KeyboardShortcutData>& GetShortcutsNotPresentInMainMenu() {
     {true, false, false, true,  kVK_ANSI_L,             IDC_SHOW_DOWNLOADS},
     {true,  true,  false, false, kVK_ANSI_C,            IDC_DEV_TOOLS_INSPECT},
     {true,  false,  false, true, kVK_ANSI_C,            IDC_DEV_TOOLS_INSPECT},
+
+    {true, false, false, true, kVK_UpArrow,             IDC_FOCUS_NEXT_PANE},
+    {true, true, false, true,  kVK_UpArrow,             IDC_FOCUS_PREVIOUS_PANE},
   });
   // clang-format on
   return *keys;

chrome/browser/global_keyboard_shortcuts_mac_unittest.mm

@@ -32,6 +32,19 @@ int CommandForKeys(bool command_key,
   if (opt_key)
     modifierFlags |= NSAlternateKeyMask;
 
+  switch (vkey_code) {
+    case kVK_UpArrow:
+    case kVK_DownArrow:
+    case kVK_LeftArrow:
+    case kVK_RightArrow:
+      // Docs say this is set whenever a key came from the numpad *or* the arrow
+      // keys.
+      modifierFlags |= NSEventModifierFlagNumericPad;
+      break;
+    default:
+      break;
+  }
+
   unichar shifted_character;
   unichar character;
   int result = ui::MacKeyCodeForWindowsKeyCode(