On opencv.js, cv.imread on a canvas element first gets the 2d context by getContext('2d').
This produces the warning on Chrome (and other webkit browsers):

Canvas2D: Multiple readback operations using getImageData are faster with the willReadFrequently attribute set to true.
See: https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-will-read-frequently

The link: https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-will-read-frequently
The correct syntax: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext#syntax
The compatibility: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext#browser_compatibility

In this PR i added the willReadFrequently attribute and checked on Chrome (compatible), Safari (not compatible) and other browsers.
On compatible browsers the warning is avoided and imread should work faster.
On non-compatible browsers the change has no effect and imread works as before.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake