[DOMStorage] Enable Onion-Soup codepath

This re-enables the onion-soup codepath, which was disabled due to
crashes (hangs) on stable. There is now more debugging info being
collected, so when we see crashes on Canary, we can figure out the
cause.

R=mek@chromium.org

Bug: 927534
Change-Id: I6f12acfd074e9931d1654e66a6a17e8ee002166e
Reviewed-on: https://chromium-review.googlesource.com/c/1456652Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630065}

third_party/blink/common/features.cc

@@ -83,7 +83,7 @@ const base::Feature kOffMainThreadSharedWorkerScriptFetch{
 
 // Onion souping for all DOMStorage. https://crbug.com/781870
 const base::Feature kOnionSoupDOMStorage{"OnionSoupDOMStorage",
-                                         base::FEATURE_DISABLED_BY_DEFAULT};
+                                         base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enable browser-initiated dedicated worker script loading
 // (PlzDedicatedWorker). https://crbug.com/906991