Not sure how come this is not hidden behind an experimental flag.  That is a mistake. That should be fixed immediately.

This will have to be standardized in the near future since it is required for feature detection, as part of the Canvas ColorSpace proposal.

Thanks!
yeah please make sure to include a spec link in the idl file once it is standardized. 

OMG. It's been in Chrome since 2013!

https://chromium.googlesource.com/chromium/src/+/d1551f0663ddb783db3d309d79145fd0df2221bd

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b1d2e017aef891e5d9bbd4d58b22fe9b08c46db3

commit b1d2e017aef891e5d9bbd4d58b22fe9b08c46db3
Author: zakerinasab <zakerinasab@chromium.org>
Date: Tue Mar 07 18:56:56 2017

Measure CanvasRenderingContext2D.getContextAttributes() usage

To make sure that we can put the unintentionally exposed
CanvasRenderingContext2D.getContextAttributes() behind a flag
without breaking websites out there, we need to measure its usage.

BUG= 696005 

Review-Url: https://codereview.chromium.org/2718783002
Cr-Commit-Position: refs/heads/master@{#455163}

[modify] https://crrev.com/b1d2e017aef891e5d9bbd4d58b22fe9b08c46db3/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/b1d2e017aef891e5d9bbd4d58b22fe9b08c46db3/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.idl
[modify] https://crrev.com/b1d2e017aef891e5d9bbd4d58b22fe9b08c46db3/tools/metrics/histograms/histograms.xml

Looking at the usage results, the usage results for all releases and all operating systems is zero, except for Canary on Android (https://uma.googleplex.com/p/chrome/usage/?sid=95ada9f8592140de4dd891de850752d1) which gives 0.003309% on M59 only at March 18, 2017. I'm not sure how this has happened, as the usage metric seems to be zero on all the other days.

On a related note, here is what is written in MDN page for CanvasRenderingContext2D (https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D):

Blink only

CanvasRenderingContext2D.getContextAttributes()
Inspired by the same WebGLRenderingContext method it returns an Canvas2DContextAttributes object that contains the attributes "storage" to indicate which storage is used ("persistent" by default) and the attribute "alpha" (true by default) to indicate that transparency is used in the canvas.

May be this is where the unintentional exposure has got public.

Yeah... we need to standardize this before going any further with it.  This API will be essential for feature detection with canvas color space support. In need to be spec'ed in a very specific way that states that the returned object contains only settings that are in effect and not settings that we ignored by the User Agent.

Could you edit the MDN page to remove references to this method? (FWIW, anyone with a GitHub account can edit MDN)

I removed the reference from MDN page. A CL is uploaded to put CanvasRenderingContext2D#getContextAttributes behind the experimental canvas features flag.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/47093fe643aa8f6a14a8800f73a4939e4be5a69f

commit 47093fe643aa8f6a14a8800f73a4939e4be5a69f
Author: zakerinasab <zakerinasab@chromium.org>
Date: Fri Apr 21 18:04:30 2017

Put CanvasRenderingContext2D#getContextAttributes behind flag

CanvasRenderingContext2D#getContextAttributes was unintentionally exposed in Chrome
back in 2013, while it should have been behind ExperimentalCanvasFeatures flag. This
CL puts the method behind a flag. This is safe as according to GetCanvas2DContextAttributes
usage histogram no one is using it at this time.

BUG= 696005 

Review-Url: https://codereview.chromium.org/2801733003
Cr-Commit-Position: refs/heads/master@{#466387}

[modify] https://crrev.com/47093fe643aa8f6a14a8800f73a4939e4be5a69f/third_party/WebKit/LayoutTests/platform/mac/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/47093fe643aa8f6a14a8800f73a4939e4be5a69f/third_party/WebKit/LayoutTests/platform/win/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/47093fe643aa8f6a14a8800f73a4939e4be5a69f/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.idl