[camera_web] Fix getCapabilities not supported error thrown when selecting a camera on Firefox

[bselwe]

Fixed getCapabilities not supported error thrown when selecting a camera on Firefox.

Before, the method was always called and an exception was thrown on Firefox with its runtime type checked against JSNoSuchMethodError. This caused issues as the runtime type was different depending on the build mode (release, profile, staging). Now, the method is called only if it is available on the video track (checked with JsUtil.hasProperty).

Part of flutter/flutter#45297.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman @felangel

[ditman]

This is a change for the better. RuntimeTypes in the web are rarely usable.