Reland "[SessionStorageS13N] Enabling mojo SessionStorage by default"

This is a reland of 1bd58466.

I want to get this feature back on in Canary so I can continue to see
crashes in the wild - there are still no crashes reproduced in tests or
reports.

TBR=jam@chromium.org
TBR=mek@chromium.org

Original change's description:
> [SessionStorageS13N] Enabling mojo SessionStorage by default
>
> Bug: 716490
> Change-Id: I08c9bc35d1e9f8f86adc6c88fb7ab8069bcf7e8f
> Reviewed-on: https://chromium-review.googlesource.com/1072918
> Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Commit-Queue: Daniel Murphy <dmurph@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#563169}

Bug: 716490
Change-Id: I6c91ffb8302ea9bead0809ef293f9a007edd608c
Reviewed-on: https://chromium-review.googlesource.com/1145201
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577010}

content/public/common/content_features.cc

@@ -239,7 +239,7 @@ const base::Feature kModuleScriptsImportMetaUrl{
 
 // Mojo-based Session Storage.
 const base::Feature kMojoSessionStorage{"MojoSessionStorage",
-                                        base::FEATURE_DISABLED_BY_DEFAULT};
+                                        base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables/disables the video capture service.
 const base::Feature kMojoVideoCapture {