[cross_file] [web] Separate "Save As" implementation details from XFile web class

[raju-muliyashiya]

fixes Issue #91400

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. You must list at least one issue.

Pre-Review Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] page, which explains my responsibilities.
• [x] I read and followed the [relevant style guides] and ran [the auto-formatter].
• [x] I signed the [CLA].
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I [linked to at least one issue that this PR fixes] in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or I have commented below to indicate which [version change exemption] this PR falls under[^1].
• [x] I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under[^1].
• [x] I updated/added any relevant documentation (doc comments with ///).
• [x] I added updated tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• [x] All existing and new tests are passing.

[raju-muliyashiya]

@ditman