Using the per-revision bisect providing the bisect results,
Good build : 64.0.3249.0 (Revision : 511319)
Bad build : 64.0.3250.0 (Revision : 511680)

You are probably looking for a change made after 511610 (known good), but no later than 511611 (first known bad).

CHANGELOG URL: 
https://chromium.googlesource.com/chromium/src/+log/3a39697331644a941b6501db7907c6942c7d8b3a..7f95c8453a369c61e3c2bf23a7fb7c8ea60b1e6b

Suspect : https://chromium.googlesource.com/chromium/src/+/7f95c8453a369c61e3c2bf23a7fb7c8ea60b1e6b

@dgozman : Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.

Note : 
1. Above issue is also observed on Mac(10.12.6) and Linux(14.04 LTS) OS.
2. This issue can also be reproduced by changing devtools theme from 'Light' to 'Dark'.

Thank you!

Tagging with blocker label, please undo or reduce priority if not the case.

Thanks.!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7a955d2eaaa66336268b5e91118cd812dc28664d

commit 7a955d2eaaa66336268b5e91118cd812dc28664d
Author: Dmitry Gozman <dgozman@chromium.org>
Date: Thu Oct 26 22:32:48 2017

[DevTools] Only recreate DevToolsFrontendHost if the frame has changed

We used to recreate DevToolsFrontendHost on any navigation and rely
on the fact that destroying old instance does not cleanup in the
renderer. This has changed with migration to mojo, where we do
properly cleanup.

Bug:  776009 ,  778525 
Change-Id: I23c8a9d959e92eae394ac5029a0ac1b160363654
Reviewed-on: https://chromium-review.googlesource.com/740087
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511988}
[modify] https://crrev.com/7a955d2eaaa66336268b5e91118cd812dc28664d/chrome/browser/devtools/devtools_ui_bindings.cc