Reland "[SessionStorageS13N] Enabling mojo SessionStorage by default"

This is a reland of 1bd58466

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: Ieae476bcb794047f5f57760eeb3e2e811838bc03
Reviewed-on: https://chromium-review.googlesource.com/1110777Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569786}

content/public/common/content_features.cc

@@ -233,7 +233,7 @@ 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 {