Good idea. Either that, or adding an IsValid() to the struct itself?

That would work as well, yeah.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4c9e1d6dda390a3098071eab7d4282f30e462256

commit 4c9e1d6dda390a3098071eab7d4282f30e462256
Author: Alexandr Ilin <alexilin@chromium.org>
Date: Tue May 22 23:42:50 2018

Add IsValid() to the base::MappedReadOnlyRegion

base::MappedReadOnlyRegion is a struct containing both a mapping and a
region. It was not always clear what the best way to check for error.
This CL adds a IsValid() method to this struct that allows to easily
perform the check.

Also this CL makes it explicit that base::MappedReadOnlyRegion has a
mapping and a region that are either both valid or invalid.

TBR=sky@chromium.org

Bug:  845086 
Change-Id: Ia61b12816167592798fc6fcb8e49f09e0198ef48
Reviewed-on: https://chromium-review.googlesource.com/1068930
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560837}
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/base/memory/read_only_shared_memory_region.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/base/memory/read_only_shared_memory_region.h
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/chrome/browser/printing/cloud_print/privet_http_unittest.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/chrome/browser/printing/pwg_raster_converter.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/chrome/browser/ui/webui/print_preview/extension_printer_handler_unittest.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/components/printing/service/pdf_compositor_service_unittest.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/components/visitedlink/browser/visitedlink_master.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/media/base/user_input_monitor.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/media/base/user_input_monitor_unittest.cc
[modify] https://crrev.com/4c9e1d6dda390a3098071eab7d4282f30e462256/services/audio/user_input_monitor_unittest.cc

Thanks for the quick patch

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/556eead9ce1e5f4b16db4db65a21ae822fb01a73

commit 556eead9ce1e5f4b16db4db65a21ae822fb01a73
Author: Lei Zhang <thestig@chromium.org>
Date: Wed May 23 16:49:27 2018

Use base::MappedReadOnlyRegion::IsValid() in printing code.

BUG= 845086 

Change-Id: I27c10824e2b5b484477592e394b171c755767ff2
Reviewed-on: https://chromium-review.googlesource.com/1067750
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561122}
[modify] https://crrev.com/556eead9ce1e5f4b16db4db65a21ae822fb01a73/chrome/services/printing/pdf_to_emf_converter.cc
[modify] https://crrev.com/556eead9ce1e5f4b16db4db65a21ae822fb01a73/chrome/services/printing/pdf_to_pwg_raster_converter.cc
[modify] https://crrev.com/556eead9ce1e5f4b16db4db65a21ae822fb01a73/components/printing/service/pdf_compositor_impl.cc