Revert "Avoid explicit conversion for CoreAccountId on desktop platforms."

This reverts commit 75be7e87.

Reason for revert: breaks compilation on fuchsia: https://ci.chromium.org/p/chromium/builders/ci/Fuchsia%20ARM64/90020

Original change's description:
> Avoid explicit conversion for CoreAccountId on desktop platforms.
> 
> Bug: 959157
> Change-Id: I8d6a6c80a4f2aea5a84264f26b4047ca46f6e687
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904173
> Reviewed-by: Tanmoy Mollik <triploblastic@chromium.org>
> Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#713817}

TBR=msarda@chromium.org,triploblastic@chromium.org

Change-Id: Ic950b9b7b4bd800b0d7c71c4ab46f17f68922137
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 959157
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906307Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713819}

chrome/browser/signin/signin_util_win.cc

@@ -65,7 +65,7 @@ std::string DecryptRefreshToken(const std::string& cipher_text) {
 // Finish the process of import credentials.  This is either called directly
 // from ImportCredentialsFromProvider() if a browser window for the profile is
 // already available or is delayed until a browser can first be opened.
-void FinishImportCredentialsFromProvider(const CoreAccountId& account_id,
+void FinishImportCredentialsFromProvider(const std::string& account_id,
                                          Browser* browser,
                                          Profile* profile,
                                          Profile::CreateStatus status) {
@@ -111,7 +111,7 @@ void ImportCredentialsFromProvider(Profile* profile,
       AboutSigninInternalsFactory::GetInstance()->GetForProfile(profile);
   signin_internals->OnAuthenticationResultReceived("Credential Provider");
 
-  CoreAccountId account_id =
+  std::string account_id =
       IdentityManagerFactory::GetForProfile(profile)
           ->GetAccountsMutator()
           ->AddOrUpdateAccount(base::UTF16ToUTF8(gaia_id),

google_apis/gaia/core_account_id.cc

@@ -22,11 +22,9 @@ CoreAccountId::CoreAccountId(std::string&& id) : id(std::move(id)) {}
 
 CoreAccountId::CoreAccountId(const std::string& id) : id(id) {}
 
-#if defined(OS_CHROMEOS) || defined(OS_IOS) || defined(OS_ANDROID)
 CoreAccountId::operator std::string() const {
   return id;
 }
-#endif
 
 bool CoreAccountId::empty() const {
   return id.empty();

google_apis/gaia/core_account_id.h

@@ -9,8 +9,6 @@
 #include <string>
 #include <vector>
 
-#include "build/build_config.h"
-
 // Represent the id of an account for interaction with GAIA. It is
 // currently implicitly convertible to and from std::string to allow
 // progressive migration of the code (see https://crbug.com/959157
@@ -24,7 +22,6 @@ struct CoreAccountId {
   CoreAccountId& operator=(const CoreAccountId&);
   CoreAccountId& operator=(CoreAccountId&&) noexcept;
 
-#if defined(OS_CHROMEOS) || defined(OS_IOS) || defined(OS_ANDROID)
   // Those implicit constructor and conversion operator allow to
   // progressively migrate the code to use this struct. Removing
   // them is tracked by https://crbug.com/959161
@@ -32,11 +29,6 @@ struct CoreAccountId {
   CoreAccountId(std::string&& id);
   CoreAccountId(const std::string& id);
   operator std::string() const;
-#else
-  explicit CoreAccountId(const char* id);
-  explicit CoreAccountId(std::string&& id);
-  explicit CoreAccountId(const std::string& id);
-#endif
 
   // Checks if the account is valid or not.
   // TODO(triploblastic): Possibly rename of remove this after