Revert "Reland "[SessionStorageS13N] Enabling mojo SessionStorage by default""

This reverts commit 5916ae31.

Bug: 854102
Reason for revert: Renderers still crashing - will have to investigate further.

Original change's description:
> 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: I5c5911a5f98e84934837fb9025431a8f98bea6ba
> Reviewed-on: https://chromium-review.googlesource.com/1104825
> Reviewed-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@{#568244}

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

Change-Id: I5cb433aa9dd9acefb8cb34cb9cc7e5820b9931cf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 716490
Reviewed-on: https://chromium-review.googlesource.com/1106398Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568486}

content/public/common/content_features.cc

@@ -228,7 +228,7 @@ const base::Feature kModuleScriptsImportMetaUrl{
 
 // Mojo-based Session Storage.
 const base::Feature kMojoSessionStorage{"MojoSessionStorage",
-                                        base::FEATURE_ENABLED_BY_DEFAULT};
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enables/disables the video capture service.
 const base::Feature kMojoVideoCapture {