Add comments for milestone markers in web_feature.mojom

This was part of another CL that I didn't end up landing,
but I did the archeology so I figured I should keep the
results.

Change-Id: I7c96e2d73014315becbef6577e3a49ae614e341a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634224
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844325}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -3017,6 +3017,8 @@ enum WebFeature {
   kOBSOLETE_AddressSpacePrivateEmbeddedInPublicNonSecureContext = 3695,
   kOBSOLETE_AddressSpacePrivateEmbeddedInUnknownSecureContext = 3696,
   kOBSOLETE_AddressSpacePrivateEmbeddedInUnknownNonSecureContext = 3697,
+  // The items above roughly this point are available in the M87 branch.
+
   kThirdPartyAccess = 3698,
   kThirdPartyActivation = 3699,
   kThirdPartyAccessAndActivation = 3700,
@@ -3051,6 +3053,8 @@ enum WebFeature {
   kMediaStreamTrackProcessor = 3729,
   kAddEventListenerWithAbortSignal = 3730,
   kXRSessionRequestLightProbe = 3731,
+  // The items above roughly this point are available in the M88 branch.
+
   kBeforematchRevealedHiddenMatchable = 3732,
   kAddSourceBufferUsingConfig = 3733,
   kChangeTypeUsingConfig = 3734,