[fuchsia][events] Define DomCode to native_keycode mapping.

- Define non-zero native_keycode values under Fuchsia, derived from the
  USB Usage Code, for keys in the Keyboard Page.
- Migrate the InputEventDispatcher for Fuchsia to use the native-to-
  DomCode conversion API.
- Fix KeycodeConverter preprocessor checks around the DOM_CODE()
  definitions to #error on unsupported platforms.

events_unittests will be enabled under Fuchsia in a follow-up CL.

Bug: 829551
Change-Id: Ia34beec8b0376980e8f599266df233f6c27cc268
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020771
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735955}

ui/events/fuchsia/input_event_dispatcher.cc

@@ -15,8 +15,6 @@
 namespace ui {
 namespace {
 
-const uint32_t kUsbHidKeyboardPage = 0x07;
-
 int KeyModifiersToFlags(int modifiers) {
   int flags = 0;
   if (modifiers & fuchsia::ui::input::kModifierShift)
@@ -167,11 +165,7 @@ bool InputEventDispatcher::ProcessKeyboardEvent(
       break;
   }
 
-  // Currently KeyboardEvent doesn't specify HID Usage page. |hid_usage|
-  // field always contains values from the Keyboard page. See
-  // https://fuchsia.atlassian.net/browse/SCN-762 .
-  DomCode dom_code = KeycodeConverter::UsbKeycodeToDomCode(
-      (kUsbHidKeyboardPage << 16) | event.hid_usage);
+  DomCode dom_code = KeycodeConverter::NativeKeycodeToDomCode(event.hid_usage);
   DomKey dom_key;
   KeyboardCode key_code;
   if (!DomCodeToUsLayoutDomKey(dom_code, KeyModifiersToFlags(event.modifiers),

ui/events/keycodes/dom/keycode_converter.cc

@@ -29,9 +29,20 @@ namespace {
 #elif defined(OS_ANDROID)
 #define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
   { usb, evdev, code }
-#else
+#elif defined(OS_FUCHSIA)
+// TODO(https://bugs.fuchsia.com/23982): Fuchsia currently delivers events
+// with a USB Code but no Page specified, so only map |native_keycode| for
+// Keyboard Usage Page codes, for now.
+inline constexpr uint32_t CodeIfOnKeyboardPage(uint32_t usage) {
+  constexpr uint32_t kUsbHidKeyboardPageBase = 0x070000;
+  if ((usage & 0xffff0000) == kUsbHidKeyboardPageBase)
+    return usage & 0xffff;
+  return 0;
+}
 #define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
-  { usb, 0, code }
+  { usb, CodeIfOnKeyboardPage(usb), code }
+#else
+#error Unsupported platform
 #endif
 #define DOM_CODE_DECLARATION const KeycodeMapEntry kDomCodeMappings[] =
 #include "ui/events/keycodes/dom/dom_code_data.inc"

ui/events/keycodes/dom/keycode_converter.h

@@ -33,6 +33,7 @@ typedef struct {
   //  On Linux: XKB scancode
   //  On Windows: Windows OEM scancode
   //  On Mac: Mac keycode
+  //  On Fuchsia: 16-bit Code from the USB Keyboard Usage Page.
   int native_keycode;
 
   // The UIEvents (aka: DOM4Events) |code| value as defined in: