Does the flaky dashboard show how often this is happening? The flaky dashboard itself is a bit flaky for me at the moment. :\

Here's a try job that's hitting the issue: https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.win%2Fwin_chromium_rel_ng%2F539634%2F%2B%2Frecipes%2Fsteps%2Fbrowser_tests__with_patch_%2F0%2Flogs%2FPDFExtensionTest.PdfZoomWithoutBubble%2F0

[ RUN      ] PDFExtensionTest.PdfZoomWithoutBubble
[3320:2720:0926/014019.230:ERROR:direct_composition_surface_win.cc(1094)] Failing to detect HDR, couldn't retrieve D3D11 device from ANGLE.
[3320:2720:0926/014019.234:INFO:media_foundation_video_encode_accelerator_win.cc(335)] Windows versions earlier than 8 are not supported.
[2024:3356:0926/014019.454:WARNING:embedded_test_server.cc(229)] Request not handled. Returning 404: /favicon.ico
[2024:4632:0926/014019.688:INFO:CONSOLE(0)] "Styling master document from stylesheets defined in HTML Imports is deprecated, and is planned to be removed in M65, around March 2018. Please refer to https://goo.gl/EGXzpw for possible migration paths.", source:  (0)
[4524:1092:0926/014020.770:WARNING:resource_message_handler.cc(49)] 'Post' message handler failed to complete successfully.

For the last line, the only file by that name is ppapi/host/resource_message_handler.cc.

Ping. This is flaking try try bots and has so for months apparently, so I'm going to disable it:
https://logs.chromium.org/v/?s=chromium%2Fbb%2Ftryserver.chromium.win%2Fwin7_chromium_rel_ng%2F56169%2F%2B%2Frecipes%2Fsteps%2Fbrowser_tests__with_patch_%2F0%2Flogs%2FPDFExtensionTest.PdfZoomWithoutBubble%2F0

https://chromium-review.googlesource.com/c/chromium/src/+/802135

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ab5ae6f0c681fa992ea861f2c7fce43b240df34

commit 7ab5ae6f0c681fa992ea861f2c7fce43b240df34
Author: Dale Curtis <dalecurtis@chromium.org>
Date: Thu Nov 30 23:02:27 2017

Disable flaky PDFExtensionTest.PdfZoomWithoutBubble on Windows.

Has been flaking for over a month without action.

BUG=767427
TEST=none

Change-Id: I751f18e8a625b21c0efbcdf09446fa648fbf28e2
Reviewed-on: https://chromium-review.googlesource.com/802135
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520734}
[modify] https://crrev.com/7ab5ae6f0c681fa992ea861f2c7fce43b240df34/chrome/browser/pdf/pdf_extension_test.cc