Add flag AllowSyncXHRInPageDismissal on chrome://flags/deprecated

Visual: http://screen/mG7ARfoC0iB

Bug: 1008156
Change-Id: I6cf6cb84ef6b797162147900b978aea293f94a28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824836Reviewed-by: Yann Dago <ydago@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700305}

chrome/browser/about_flags.cc

@@ -4232,7 +4232,8 @@ const FeatureEntry kFeatureEntries[] = {
 
     {"allow-sync-xhr-in-page-dismissal",
      flag_descriptions::kAllowSyncXHRInPageDismissalName,
-     flag_descriptions::kAllowSyncXHRInPageDismissalDescription, kOsAll,
+     flag_descriptions::kAllowSyncXHRInPageDismissalDescription,
+     kOsAll | kDeprecated,
      FEATURE_VALUE_TYPE(blink::features::kAllowSyncXHRInPageDismissal)},
 
     {"form-controls-refresh", flag_descriptions::kFormControlsRefreshName,