[WebExtensions] Deprecate Native Client info in runtime API (#29491)

Add notes to runtime.PlatformInfo.nacl_arch and runtime.PlatformNaclArch
describing their deprecation. Google Chrome plans to run a deprecation
experiment removing runtime.PlatformInfo.nacl_arch on all platforms.
Shortly after conclussion of runtime.PlatformInfo.nacl_arch removal experiment
Chrome plans to remove enum runtime.PlatformNaclArch without any experiment.

Details:
https://chromium-review.googlesource.com/c/chromium/src/+/7090141

Author: bershanskiy

webextensions/api/runtime.json

@@ -360,7 +360,8 @@
             "__compat": {
               "support": {
                 "chrome": {
-                  "version_added": "≤58"
+                  "version_added": "28",
+                  "notes": "This attribute is deprecated and is being removed from Google Chrome, following removal of Google Native Client."
                 },
                 "edge": "mirror",
                 "firefox": {
@@ -381,7 +382,8 @@
             "mdn_url": "https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/API/runtime/PlatformNaclArch",
             "support": {
               "chrome": {
-                "version_added": "≤58"
+                "version_added": "28",
+                "notes": "This enum is deprecated and is being removed from Google Chrome, following removal of Google Native Client."
               },
               "edge": "mirror",
               "firefox": {