webml: Map model loader content feature to Blink flag

This change makes it easier to enable the prototype Model Loader API by
mapping the content::kEnableMachineLearningModelLoaderWebPlatformApi
feature flag to the MachineLearningModelLoader Blink feature flag so
that they can be enabled at the same time through about:flags instead of
requiring all experimental Blink features to be enabled.

Bug: b/259438440
Change-Id: I653fe03e3c55136a1a5540cf17e58bb655f38ae2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4945110
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210596}