Revert "Use thread_local for WTF::IsMainThread."

This reverts commit 7fa769eb.

Bug: 927285
Change-Id: I003834a44cc27075be7071fca091869bb85e427f
Reviewed-on: https://chromium-review.googlesource.com/c/1448680Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628209}

styleguide/c++/c++11.html

@@ -318,14 +318,6 @@ template &lt;typename T&gt;<br/>void Function(T&amp;&amp; t) { ... }</code></td>
 <td>Use only where it considerably improves readability. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/OQyYSfH9m2M">Discussion thread</a>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/Lkp0nubVd0Q">Another discussion thread</a></td>
 </tr>
 
-<tr>
-<td>thread_local storage class</td>
-<td><code>thread_local int foo = 1;</code></td>
-<td>Puts variables into thread local storage.</td>
-<td><a href="http://en.cppreference.com/w/cpp/language/storage_duration">Storage duration</a></td>
-<td>Allowed only for POD data. (<a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/2msN8k3Xzgs">discussion</a>, <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/h7O5BdtWCZw">fork</a>). Use <a href="https://cs.chromium.org/chromium/src/base/threading/sequence_local_storage_slot.h">SequenceLocalStorageSlot</a> for sequence support, and <a href="https://cs.chromium.org/chromium/src/base/threading/thread_local.h">ThreadLocal</a>/<a href="https://cs.chromium.org/chromium/src/base/threading/thread_local_storage.h">ThreadLocalStorage</a> for other cases.</td>
-</tr>
-
 <tr>
 <td>Type Aliases ("using" instead of "typedef")</td>
 <td><code>using <i>new_alias</i> = <i>typename</i></code></td>
@@ -784,6 +776,14 @@ std::move(t).f();&nbsp;&nbsp;// second</code></td>
 <td>Banned in the <a href="https://google.github.io/styleguide/cppguide.html#Operator_Overloading">Google Style Guide</a>.</td>
 </tr>
 
+<tr>
+<td>thread_local storage class</td>
+<td><code>thread_local int foo = 1;</code></td>
+<td>Puts variables into thread local storage.</td>
+<td><a href="http://en.cppreference.com/w/cpp/language/storage_duration">Storage duration</a></td>
+<td>Some surprising effects on Mac (<a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/2msN8k3Xzgs">discussion</a>, <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/h7O5BdtWCZw">fork</a>). Use <a href="https://cs.chromium.org/chromium/src/base/threading/sequence_local_storage_slot.h">SequenceLocalStorageSlot</a> for sequence support, and <a href="https://cs.chromium.org/chromium/src/base/threading/thread_local.h">ThreadLocal</a>/<a href="https://cs.chromium.org/chromium/src/base/threading/thread_local_storage.h">ThreadLocalStorage</a> otherwise.</td>
+</tr>
+
 </tbody>
 </table>
 

third_party/blink/renderer/platform/wtf/thread_specific.h

@@ -87,10 +87,7 @@ inline void ThreadSpecific<T>::Destroy(void* ptr) {
   // longer has a graceful shutdown sequence. Be careful to call this function
   // (which can be re-entrant) while the pointer is still set, to avoid lazily
   // allocating WTFThreadData after it is destroyed.
-  // This check cannot be performed using WTF::IsMainThread, as it uses TLS to
-  // store thread information, and we can't rely on the destruction order of
-  // TLS variables.
-  if (CurrentThread() == g_main_thread_identifier)
+  if (IsMainThread())
     return;
 
   // The memory was allocated via Partitions::FastZeroedMalloc, and then the

third_party/blink/renderer/platform/wtf/wtf.cc

@@ -49,15 +49,6 @@ bool g_initialized;
 void (*g_call_on_main_thread_function)(MainThreadFunction, void*);
 base::PlatformThreadId g_main_thread_identifier;
 
-#if defined(COMPONENT_BUILD) && defined(OS_WIN)
-static thread_local bool g_is_main_thread = false;
-bool IsMainThread() {
-  return g_is_main_thread;
-}
-#else
-thread_local bool g_is_main_thread = false;
-#endif
-
 namespace internal {
 
 void CallOnMainThread(MainThreadFunction* function, void* context) {
@@ -66,13 +57,16 @@ void CallOnMainThread(MainThreadFunction* function, void* context) {
 
 }  // namespace internal
 
+bool IsMainThread() {
+  return CurrentThread() == g_main_thread_identifier;
+}
+
 void Initialize(void (*call_on_main_thread_function)(MainThreadFunction,
                                                      void*)) {
   // WTF, and Blink in general, cannot handle being re-initialized.
   // Make that explicit here.
   CHECK(!g_initialized);
   g_initialized = true;
-  g_is_main_thread = true;
   g_main_thread_identifier = CurrentThread();
 
   WTFThreadData::Initialize();

third_party/blink/renderer/platform/wtf/wtf.h

@@ -31,7 +31,6 @@
 #ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_WTF_WTF_H_
 #define THIRD_PARTY_BLINK_RENDERER_PLATFORM_WTF_WTF_H_
 
-#include "build/build_config.h"
 #include "third_party/blink/renderer/platform/wtf/compiler.h"
 #include "third_party/blink/renderer/platform/wtf/threading.h"
 #include "third_party/blink/renderer/platform/wtf/wtf_export.h"
@@ -44,17 +43,7 @@ WTF_EXPORT extern base::PlatformThreadId g_main_thread_identifier;
 // This function must be called exactly once from the main thread before using
 // anything else in WTF.
 WTF_EXPORT void Initialize(void (*)(MainThreadFunction, void*));
-
-// thread_local variables can't be exported on Windows, so we use an extra
-// function call on component builds.
-#if defined(COMPONENT_BUILD) && defined(OS_WIN)
 WTF_EXPORT bool IsMainThread();
-#else
-WTF_EXPORT extern thread_local bool g_is_main_thread;
-inline bool IsMainThread() {
-  return g_is_main_thread;
-}
-#endif
 
 namespace internal {
 void CallOnMainThread(MainThreadFunction*, void* context);