My changes to these lines were all cosmetic. Sending to James for investigation.

Please specify the version number, sha or whatnot as we have no idea what code this bug was reproduced against.

It has been happening for a while and it still happening on Chromium ToT after I pulled af9b038a6bf7758c74df279bc15a77a907ee61af today.

I'm testing on cyan (Acer Chromebook R11). I open a few tabs and scroll around, my hope is to reproduce https://bugs.chromium.org/p/chromium/issues/detail?id=629521 but render_widget_host_impl.cc(1125) is the DCHECK I've encountered the most so far.

The cros image is R58-9304.0.0.

This just happened again on kevin with ToT Chrome (crrev.com/b761712bc735e72369c527fc4cd861b39e5c20cf):

[19241:19241:0307/173306.084729:FATAL:render_widget_host_impl.cc(1103)] Check failed: *is_in_gesture_scroll || (gesture_event.type() == blink::WebInputEvent::GestureFlingStart && gesture_event.sourceDevice == blink::WebGestureDevice::WebGestureDeviceTouchpad).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/74937dc645b191514bde662169aee3b29074b39d

commit 74937dc645b191514bde662169aee3b29074b39d
Author: dcastagna <dcastagna@chromium.org>
Date: Wed Mar 08 22:40:08 2017

Temporarily disable GestureFlingStart input DCHECK.

This DCHECK causes the browser process to crash and makes debuggining on
a debug build (dchecks on) really tedious on ChromeOS.

This CL temporarily disables the DCHECK while we work on a real fix.

BUG= 695187 

Review-Url: https://codereview.chromium.org/2737943002
Cr-Commit-Position: refs/heads/master@{#455564}

[modify] https://crrev.com/74937dc645b191514bde662169aee3b29074b39d/content/browser/renderer_host/render_widget_host_impl.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cff98375334e29507d061fd93d4e9146c785f43f

commit cff98375334e29507d061fd93d4e9146c785f43f
Author: wjmaclean <wjmaclean@chromium.org>
Date: Fri Apr 28 18:56:55 2017

Forward GestureFlingStart in OOPIF, re-enable DCHECK.

Since GestureScrolls can be ended by either GestureScrollEnd or
GestureFlingStart, it seems wrong that OOPIF scroll bubbling, in
RenderWidgetHostViewChildFrame/CrossProcessFrameConnector doesn't
bubble GestureFlingStart.

This CL modifies the bubbling code to include bubbling of
GestureFlingStart, and re-enables a related DHECK in
RenderWidgetHostImpl.

BUG= 695187 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2842203002
Cr-Commit-Position: refs/heads/master@{#468079}

[modify] https://crrev.com/cff98375334e29507d061fd93d4e9146c785f43f/content/browser/frame_host/cross_process_frame_connector.cc
[modify] https://crrev.com/cff98375334e29507d061fd93d4e9146c785f43f/content/browser/frame_host/render_widget_host_view_child_frame.cc
[modify] https://crrev.com/cff98375334e29507d061fd93d4e9146c785f43f/content/browser/renderer_host/render_widget_host_impl.cc
[modify] https://crrev.com/cff98375334e29507d061fd93d4e9146c785f43f/content/browser/renderer_host/render_widget_host_input_event_router.cc
[modify] https://crrev.com/cff98375334e29507d061fd93d4e9146c785f43f/content/browser/site_per_process_browsertest.cc

This should be fixed now. Please re-open if we start hitting this again.