Const-correctness in third_party/blink/renderer/core/html/forms/email_input_type.cc.

Change-Id: Ibbfa3fa1b5c1dfaefefbda49c5a59293113831ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1646780
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666700}