clang/win: Fix style errors after https://codereview.chromium.org/1585193002

..\..\ui/events/keycodes/platform_key_map_win.h(21,1) :
    error: [chromium-style] Complex class/struct needs
           an explicit out-of-line destructor.
class EVENTS_BASE_EXPORT PlatformKeyMap {
^

..\..\ui\events\keycodes\platform_key_map_win.cc(47,21) :
    error: comparison of integers of different signs:
           'int' and 'unsigned int' [-Werror,-Wsign-compare]
  for (int i = 0; i < arraysize(modifier_flags); ++i) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~

BUG=82385
TBR=chongz

Review URL: https://codereview.chromium.org/1705053002 .

Cr-Commit-Position: refs/heads/master@{#375888}
2 files changed