Screenshot of a dialog with ShouldShowDialogTitle()=false:
https://drive.google.com/file/d/1GR0l5EmFtX9R_ywa9vepuYkVt5_irPUT/view?usp=sharing

Screenshot after the fix:
https://drive.google.com/file/d/1w6ypbHR446CxU5oU9rPGGRQ8-4bFaGr1/view?usp=sharing

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2cbab899749862da71732290935efdcbc97ecc19

commit 2cbab899749862da71732290935efdcbc97ecc19
Author: Yue Li <updowndota@chromium.org>
Date: Wed Apr 11 08:45:03 2018

Handle ShouldShowDialogTitle for WebDialogDelegate

Currently WebDialogDelegate cannot hide the dialog title by setting
ShouldShowDialogTitle.

Bug:  828574 
Test: Local Build
Change-Id: I2d9abc21ef108ccd97dc4667c2d4b311f6ec3501
Reviewed-on: https://chromium-review.googlesource.com/993756
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549830}
[modify] https://crrev.com/2cbab899749862da71732290935efdcbc97ecc19/chrome/browser/ui/browser_dialogs.h
[modify] https://crrev.com/2cbab899749862da71732290935efdcbc97ecc19/chrome/browser/ui/views/chrome_web_dialog_view.cc
[modify] https://crrev.com/2cbab899749862da71732290935efdcbc97ecc19/chrome/browser/ui/webui/chromeos/assistant_optin/assistant_optin_ui.cc
[modify] https://crrev.com/2cbab899749862da71732290935efdcbc97ecc19/chrome/browser/ui/webui/chromeos/system_web_dialog_delegate.cc
[modify] https://crrev.com/2cbab899749862da71732290935efdcbc97ecc19/chrome/browser/ui/webui/chromeos/system_web_dialog_delegate.h