Looks like expectations aren't being met on particular trybots for some reason. For example, from https://chromium-swarm.appspot.com/task?id=3e52e8b272602c10&refresh=10&show_raw=1:

[4912:4404:0625/201718.154:ERROR:dump_accessibility_browsertest_base.cc(398)] Diff:
* Line Expected
- ---- --------
     1 rootWebArea pageLocation=(0, 0)
     2 ++iframe pageLocation=(0, 100) transform
     3 ++++rootWebArea pageLocation=(0, 100)
     4 ++++++iframe pageLocation=(200, 100) transform
     5 ++++++++rootWebArea pageLocation=(200, 100)
*    6 ++++++++++image pageLocation=(240, 140) pageSize=(560, 460)
     7 <-- End-of-file -->

Actual
------
rootWebArea pageLocation=(0, 0)
++iframe pageLocation=(0, 100) transform
++++rootWebArea pageLocation=(0, 100)
++++++iframe pageLocation=(200, 100) transform
++++++++rootWebArea pageLocation=(200, 100)
++++++++++image pageLocation=(240, 140)
<-- End-of-file -->

and:

[3732:1648:0625/201719.328:ERROR:dump_accessibility_browsertest_base.cc(398)] Diff:
* Line Expected
- ---- --------
     1 rootWebArea pageLocation=(0, 0)
     2 ++iframe pageLocation=(0, 100) transform
     3 ++++rootWebArea pageLocation=(0, 100)
     4 ++++++iframe pageLocation=(200, 100) transform
     5 ++++++++rootWebArea pageLocation=(200, 100)
*    6 ++++++++++image pageLocation=(240, 140) pageSize=(560, 460)
     7 <-- End-of-file -->

Actual
------
rootWebArea pageLocation=(0, 0)
++iframe pageLocation=(0, 100) transform
++++rootWebArea pageLocation=(0, 100)
++++++iframe pageLocation=(200, 100) transform
++++++++rootWebArea pageLocation=(200, 100)
++++++++++image pageLocation=(240, 140)
<-- End-of-file -->

I'll send up a CL to disable these on Windows for now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4a9fc81ede9ef29553c4b042a88c08b357db1d47

commit 4a9fc81ede9ef29553c4b042a88c08b357db1d47
Author: Greg Thompson <grt@chromium.org>
Date: Tue Jun 26 12:01:51 2018

Disable flaky accessibility tests on Windows.

DumpAccessibilityTreeTest.AccessibilityIframeTransform
DumpAccessibilityTreeTest.AccessibilityIframeTransformNested
DumpAccessibilityTreeTest.AccessibilityIframeTransformNestedCrossProcess

BUG= 856566 
TBR=dmazzoni@chromium.org

Change-Id: I8abbd6a00ab3e995d0974cfd59cb0f9b046a732d
Reviewed-on: https://chromium-review.googlesource.com/1114738
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570377}
[modify] https://crrev.com/4a9fc81ede9ef29553c4b042a88c08b357db1d47/content/browser/accessibility/dump_accessibility_tree_browsertest.cc

 Issue 856817  has been merged into this issue.

 Issue 856816  has been merged into this issue.

Dominic, do you think this could be fixed the same way as  issue 856367 ? (I was linked here from that bug.) I could write up a CL, but I wouldn't be able to try it on windows.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f1fc2f4fc916b45472cfd5951cc51a327f07a54

commit 5f1fc2f4fc916b45472cfd5951cc51a327f07a54
Author: Dominic Mazzoni <dmazzoni@chromium.org>
Date: Thu Jul 19 06:40:45 2018

Fix all of the AccessibilityIframeTransform* content_browsertests

The flakiness was all due to the main browser window not being consistently
800 x 600. This was affecting any accessibility test that was asserting
anything about the clipped bounding box of a node with a y-coordinate
greater than 600, for example.

The solution is to make the dimensions of things a bit smaller so
tests aren't more than 600 pixels tall, and to tighten up the
expectations so we aren't relying on anything that might flake.

TBR=katie@chromium.org

Bug:  856566 
Change-Id: I391a880c476d3570a97aef1732cd5f2c6de422e4
Reviewed-on: https://chromium-review.googlesource.com/1142532
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576397}
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/browser/accessibility/dump_accessibility_tree_browsertest.cc
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/frame/box.html
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-cross-process-expected-blink.txt
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-cross-process.html
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-expected-blink.txt
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-expected-win.txt
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-nested-cross-process-expected-blink.txt
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-nested-cross-process.html
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-nested-expected-blink.txt
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform-nested.html
[modify] https://crrev.com/5f1fc2f4fc916b45472cfd5951cc51a327f07a54/content/test/data/accessibility/html/iframe-transform.html