Revert of offset namespace ids (patchset #7 id:120001 of...

Revert of offset namespace ids (patchset #7 id:120001 of https://codereview.chromium.org/881253004/)

Reason for revert:
Tests now only work if they ran in the "right" order. DOMStorageContextImplTest.Basic fails if the tests are shuffled:

content_unittests --gtest_filter=DOMStorageContextImplTest.*:DOMStorageAreaTest.* --gtest_shuffle

Original issue's description:
> [DOMStorage] Offset the session storage namespace ids generated by different contexts to help identify when an id from one is mistakenly used in another.
>
> Committed: https://crrev.com/36dcef3da5317365ab95e50a5f6b7fa98ada1d5e
> Cr-Commit-Position: refs/heads/master@{#314256}

TBR=jsbell@chromium.org,michaeln@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/896733002

Cr-Commit-Position: refs/heads/master@{#314274}

content/browser/dom_storage/dom_storage_context_impl.cc

@@ -4,8 +4,6 @@
 
 #include "content/browser/dom_storage/dom_storage_context_impl.h"
 
-#include <stdlib.h>
-
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_enumerator.h"
@@ -28,10 +26,6 @@ namespace content {
 
 static const int kSessionStoraceScavengingSeconds = 60;
 
-// Offset the session storage namespace ids generated by different contexts
-// to help identify when an id from one is mistakenly used in another.
-static int g_session_id_offset_sequence = 0;
-
 DOMStorageContextImpl::DOMStorageContextImpl(
     const base::FilePath& localstorage_directory,
     const base::FilePath& sessionstorage_directory,
@@ -40,7 +34,6 @@ DOMStorageContextImpl::DOMStorageContextImpl(
     : localstorage_directory_(localstorage_directory),
       sessionstorage_directory_(sessionstorage_directory),
       task_runner_(task_runner),
-      session_id_offset_(abs((g_session_id_offset_sequence++ % 10)) * 1000),
       is_shutdown_(false),
       force_keep_session_state_(false),
       special_storage_policy_(special_storage_policy),
@@ -234,10 +227,6 @@ void DOMStorageContextImpl::NotifyAreaCleared(
       OnDOMStorageAreaCleared(area, page_url));
 }
 
-int64 DOMStorageContextImpl::AllocateSessionId() {
-  return session_id_sequence_.GetNext() + session_id_offset_;
-}
-
 std::string DOMStorageContextImpl::AllocatePersistentSessionId() {
   std::string guid = base::GenerateGUID();
   std::replace(guid.begin(), guid.end(), '-', '_');

content/browser/dom_storage/dom_storage_context_impl.h

@@ -147,7 +147,10 @@ class CONTENT_EXPORT DOMStorageContextImpl
       const GURL& page_url);
 
   // May be called on any thread.
-  int64 AllocateSessionId();
+  int64 AllocateSessionId() {
+    return session_id_sequence_.GetNext();
+  }
+
   std::string AllocatePersistentSessionId();
 
   // Must be called on the background thread.
@@ -204,10 +207,7 @@ class CONTENT_EXPORT DOMStorageContextImpl
 
   // We use a 32 bit identifier for per tab storage sessions.
   // At a tab per second, this range is large enough for 68 years.
-  // The offset is to more quickly detect the error condition where
-  // an id related to one context is mistakenly used in another.
   base::AtomicSequenceNumber session_id_sequence_;
-  const int session_id_offset_;
 
   bool is_shutdown_;
   bool force_keep_session_state_;