Remove BLINK_PLATFORM_EXPORT from WebWaitableEvent

It's not a class exported from blink_platform but to be
implemented by the embedder.

BUG=334263
TEST=build

Review URL: https://codereview.chromium.org/141573004

git-svn-id: svn://svn.chromium.org/blink/trunk@165314 bbb929c8-8fbe-4397-9dbb-9b2b20218538