Revert "[SessionStorage] Renderer piping & browser stubs for mojofication."

This reverts commit ab3de2ef31b1bcf76140c6ba61fbc5514ebf7749.

Reason for revert: ODR violation - content::SessionStorageNamespace already exists.

Original change's description:
> [SessionStorage] Renderer piping & browser stubs for mojofication.
> 
> This change adds a feature flag and the piping to hook up & run a mojo
> implementation of SessionStorage. Nothing is implemented yet and all
> calls trip a NOTREACHED - this is just the setup for the renderer side
> of the feature.
> 
> Bug: 716490
> Change-Id: Id3ad5cf3fa1e27bf7e5ef9369818d94a1a5364d3
> Reviewed-on: https://chromium-review.googlesource.com/767287
> Reviewed-by: Oliver Chang <ochang@chromium.org>
> 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@{#517619}

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

Change-Id: I51f09c1bc483ec16e3ae038d9117523c788b9442
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 716490
Reviewed-on: https://chromium-review.googlesource.com/777411
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517718}
31 files changed