See test case here: https://codereview.chromium.org/2732913005/

The main issue is that if the next/previous element in focus order relative to the webview is focused and the webview is focused by the browser (clicking in the webview instead of tabbing in), then it is impossible to tab out of the webview in that direction

For example: <button id=a><webview><button id=b>. Start with b focused -> click the webview -> repeated tab keypressed. In this case focus never moves out of the webview and onto b.

Nothing happens in the embedder which should be refocusing b due to https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/page/FocusController.cpp?sq=package:chromium&dr&l=1001