Retrieve Mozc's installation dir from registry

This is a preparation to switch Mozc's installation directory from

  %ProgramFiles(x86)%\Mozc

into

  %ProgramFiles%\Mozc

without user observable downtime (google#1086).

Currently the directory

  %ProgramFiles(x86)%\Mozc

is effectively hard coded in 'mozc_tip{32,64}.dll'. This means if older
versions of 'mozc_tip{32,64}.dll' continue assuming the previous
installation directory until it is unloaded or the process is restarted.

To avoid such mismatch, with this commit

  SystemUtil::GetServerDirectory()

starts dynamically retrieving Mozc's installation directory from
registry by checking COM entries of 'mozc_tip64.dll'. If it fails, we
fall back to the previous behavior.

Note that this is the first step of the migration and we cannot go to
the next step until 'mozc_tip{32,64}.dll' with this commit is deployed
to users.

src/base/system_util.cc

@@ -409,11 +409,48 @@ class ProgramFilesX86Cache {
   HRESULT result_;
   std::string path_;
 };
+
+constexpr wchar_t kMozcTipClsid[] =
+    L"SOFTWARE\\Classes\\CLSID\\"
+#ifdef GOOGLE_JAPANESE_INPUT_BUILD
+    L"{D5A86FD5-5308-47EA-AD16-9C4EB160EC3C}"
+#else   // GOOGLE_JAPANESE_INPUT_BUILD
+    L"{10A67BC8-22FA-4A59-90DC-2546652C56BF}"
+#endif  // GOOGLE_JAPANESE_INPUT_BUILD
+    L"\\InprocServer32";
+
+std::string GetMozcInstallDirFromRegistry() {
+  // TSF requires the path of "mozc_tip64.dll" to be registered in the registry,
+  // which tells us Mozc's installation directory.
+  HKEY key = nullptr;
+  LSTATUS result =::RegOpenKeyExW(
+      HKEY_LOCAL_MACHINE, kMozcTipClsid, 0, KEY_READ | KEY_WOW64_64KEY, &key);
+  if (result != ERROR_SUCCESS) {
+    return "";
+  }
+
+  DWORD type = 0;
+  wchar_t buffer[MAX_PATH] = {};
+  DWORD buffer_size = sizeof(buffer);
+  result = ::RegQueryValueExW(
+      key, nullptr, nullptr, &type, reinterpret_cast<LPBYTE>(buffer),
+      &buffer_size);
+  ::RegCloseKey(key);
+  if (result != ERROR_SUCCESS || type != REG_SZ) {
+    return "";
+  }
+  return FileUtil::Dirname(win32::WideToUtf8(buffer));
+}
+
 }  // namespace
 #endif  // _WIN32
 
 std::string SystemUtil::GetServerDirectory() {
 #ifdef _WIN32
+  const std::string install_dir_from_registry = GetMozcInstallDirFromRegistry();
+  if (!install_dir_from_registry.empty()) {
+    return install_dir_from_registry;
+  }
   DCHECK(SUCCEEDED(Singleton<ProgramFilesX86Cache>::get()->result()));
 #if defined(GOOGLE_JAPANESE_INPUT_BUILD)
   return FileUtil::JoinPath(