Chrome Version: 68.0.3426.0
OS: Linux, probably all desktop platforms.

What steps will reproduce the problem?
(1) Open a PDF with forms, e.g. https://www.irs.gov/pub/irs-pdf/fw4.pdf
(2) Put something into the clipboard buffer
(3) Select a text form field in the PDF, press ctrl + v.

What is the expected result?

No DCHECK failures.

What happens instead?

[1:1:0509/195945.468780:FATAL:clipboard_struct_traits.h(18)] Check failed: false. 
#0 0x7fc8a3b289bd base::debug::StackTrace::StackTrace()
#1 0x7fc8a38524bc base::debug::StackTrace::StackTrace()
#2 0x7fc8a38c3faa logging::LogMessage::~LogMessage()
#3 0x7fc898e03afb mojo::EnumTraits<>::ToMojom()
#4 0x7fc898e03a67 mojo::internal::Serializer<>::Serialize()
#5 0x7fc898df7e82 _ZN4mojo8internal9SerializeIN5blink5mojom15ClipboardBufferERN2ui13ClipboardTypeEJPiELPv0EEEvOT0_DpOT1_
#6 0x7fc898de3575 blink::mojom::ClipboardHostProxy::ReadText()
#7 0x7fc89ef0e28d content::PepperWebPluginImpl::ExecuteEditCommand()
...