Using the per-revision bisect providing the bisect results,
Good build: 55.0.2868.0 (Revision: 420217).
Bad build: 55.0.2871.0 (Revision: 420831).

You are probably looking for a change made after 420536 (known good), but no later than 420537 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/333a7dd429e44a6562112fc29c667474fa2e92f9..4b7467a5a8e36f6d59cbc2df29be42e9c12ec45b

Note: Adding RB Label as this is a recent Regression. Please remove if not required.
@ananta -- 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.
Thank You.

ananta@: Please take a look at this and update accordingly.

Note: Issue is still reproducible on the latest Dev(55.0.2879.0) as tested on Linux Ubuntu 14.04.

Friendly ping to get an update on this issue.

Can we get an update on this issue.

Not sure what I can do here as I don't understand PlzNavigate.

Fixed in this patch https://codereview.chromium.org/2418963002/

[Auto-generated comment by a script] We noticed that this issue is targeted for M-55; it appears the fix may have landed after branch point, meaning a merge might be required. Please confirm if a merge is required here - if so add Merge-Request-55 label, otherwise remove Merge-TBD label. Thanks.

CL listed at #7 landed after M55 branch on oct 6th. ananta@, please request a merge to M55 ASAP as this bug is reported as M55 stable blocker. Thank you.

Approving merge to M55 branch 2883 for CL listed at #7 (https://codereview.chromium.org/2418963002/). Please merge ASAP before 4:00 PM PT if possible so we can pick it up for tomorrow's Beta release.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e7dac575925f858b0b344798e776c859f7c6076b

commit e7dac575925f858b0b344798e776c859f7c6076b
Author: Anantanarayanan G Iyengar <ananta@chromium.org>
Date: Tue Nov 01 19:27:46 2016

Don't abort the MimeHandlerStreamManager stream if we receive a RenderProcessHostChanged notification for a null old host.

Merging to M55

This indicates that a subframe is being created. If we ignore  the old_host here and set
the new host pointer in the MimeHandlerStreamManager::EmbedderObserver class, this causes
the stream to be aborted prematurely leading to embedded PDFs not loading.

This should fix  bug 650985 

BUG= 650985 

Review-Url: https://codereview.chromium.org/2418963002
Cr-Commit-Position: refs/heads/master@{#425498}
(cherry picked from commit b8cfa3b172b2741635247a8541e3251688aaa04f)

Review URL: https://codereview.chromium.org/2465223003 .

Cr-Commit-Position: refs/branch-heads/2883@{#407}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/e7dac575925f858b0b344798e776c859f7c6076b/content/browser/web_contents/web_contents_impl.cc
[modify] https://crrev.com/e7dac575925f858b0b344798e776c859f7c6076b/extensions/browser/guest_view/mime_handler_view/mime_handler_stream_manager.cc