The issue exists since a long time ago (I verified for as far back as 50.0.2641.0).

What steps will reproduce the problem?
(1) Navigate chrome to a PDF file.
(2) Inspect the tab (go to chrome://inspect and from pages inspect the tab).
(3) Notice the page contains an <embed>. This corresponds to a BrowserPlugin which is a WebPlugin.
(4) Edit the HTML using devtools to insert an <input/> before the <embed>
(5) Focus the <input> and type some characters.
(6) Notice the characters are inserted as expected (keyboard input works fine).
(7) Try some editing commands such as Cmd + A or try to use some IME such as Pinyin Simplified.

What is the expected result?
The editing commands work find.

What happens instead?
The editing commands do not work.

The issue seems to be with the implementation of FocusedPluginIfInputMethodSupported(LocalFrame). This method always returns a WebPlugin when there is on inside the frame.