Removing "--enable-experimental-canvas-features" flag and merge canvas features into the rest of web platform features

[xl...@chromium.org]

Currently, all the canvas experimental features are hidden behind the ExperimentalCanvasFeatures flag, whilst the rest of Blink features are all centrally managed in Runtime Enabled Features file and hidden behind the ExperimentalWebPlatformFeatures flag.

To increase feature maintainability and reduce user confusion about which flag to use when they want to try out experimental features, we think it would be better to move canvas features to be under the umbrella of Experimental Web Platform Features. As such, the "--enable-experimental-canvas-features" flag could be removed from Chromium and users can access the experimental canvas features via the "--enable-experimental-web-platform-features" flag. 

The progress of this change can be tracked on Chromium Issue 817381 and we'll make another announcement when it is done.

[xl...@chromium.org]

The "Experimental Canvas Features" flag has been removed from Chrome in the latest Canary (CL). To play around with experimental canvas features, you could turn on the "Experimental Web Platform features" flag.