Update the Estonian workaround comments.

Target date for removal of the workaround is 6 months.

BUG=532048

Change-Id: I402f75e46736936725575559cd8eb194115ab0df
Reviewed-on: https://boringssl-review.googlesource.com/5910
Reviewed-by: Adam Langley <agl@google.com>
(cherry picked from commit c71567dd50b1915ce3a72be0d63ad598d06f4879)