Remove status:experimental for blink feature AccessibilityExposeHTMLElement

This is based on the feedback in
https://docs.google.com/spreadsheets/d/1DAtdbGy7XZCC_1_IXHXZKx568pw0eDlVETCtJBexqZc/edit#gid=0.

This feature is not supposed to be enabled for web tests.

Bug: 1121839
Change-Id: I2713381e147dabf7b24090738afd1cd02e7c53b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380313Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802756}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -125,7 +125,6 @@
     },
     {
       name: "AccessibilityExposeHTMLElement",
-      status: "experimental",
     },
     {
       name: "AccessibilityObjectModel",