Quota: Rename quota_manager.{cc,h} to quota_manager_impl.{cc,h}.

This is a part of a sequence of CLs that will result in
storage::QuotaManager being renamed to storage::QuotaManagerImpl. During
the preparation for QuotaManager mojofication, code
outside //storage will access storage::QuotaManagerImpl via a new
storage::QuotaManager interface, which will be implemented by
QuotaManagerProxy. After mojofication, storage::QuotaManager will be
replaced by storage::mojom::QuotaManager, which will be implemented by
storage::QuotaManagerImpl. storage::QuotaManagerProxy will become a
thread-safe proxy around a storage::mojom::QuotaManager remote.

This CL performs the following mechanical steps.

1. Move //storage/browser/quota/quota_manager.cc to
   //storage/browser/quota/quota_manager_impl.cc
2. Move //storage/browser/quota/quota_manager.h to
   //storage/browser/quota/quota_manager_impl.h
3. Replace STORAGE_BROWSER_QUOTA_QUOTA_MANAGER_H_ with
   STORAGE_BROWSER_QUOTA_QUOTA_MANAGER_IMPL_H_
4. Replace storage/browser/quota/quota_manager.h with
   storage/browser/quota/quota_manager_impl.h
   in //storage/browser/quota/quota_manager_impl.cc

Steps 3 and 4 are the minimal changes for quota_manager_impl.{cc,h} to
meet our style guides.

The following edits were done manually:

1. Create header quota_manager.h that #includes quota_manager_impl.h.
   This avoids introducing changes to all the code that #includes
   quota_manager.h.
2. Create implementation file quota_manager.cc that #includes
   quota_manager_impl.cc. This currently verifies that quota_manager.h
   compiles standalone, and will eventually host QuotaManager interface
   code.
3. Add quota_manager_impl.{cc,h} to //storage/browser/BUILD.gn.

Renaming storage::QuotaManager to storage::QuotaManagerImpl will happen
in a follow-up CL, to reduce the cost of the code review.

Bug: 1168597
Change-Id: Ibb410e65c1b2b9f7e1c2d338d100fcac99808593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2649623
Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#847659}

storage/browser/BUILD.gn

@@ -199,6 +199,8 @@ component("browser") {
     "quota/quota_macros.h",
     "quota/quota_manager.cc",
     "quota/quota_manager.h",
+    "quota/quota_manager_impl.cc",
+    "quota/quota_manager_impl.h",
     "quota/quota_manager_proxy.cc",
     "quota/quota_manager_proxy.h",
     "quota/quota_override_handle.cc",