[date] Refactor TimezoneCache to be separate from the OS

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 160ad66bae885c7c95756e85b754d94e07d361a7..29c82bcb12b9dad63de7a3e7043153a68a9a19f9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2317,6 +2317,7 @@ v8_component("v8_libbase") {
     "src/base/safe_math_impl.h",
     "src/base/sys-info.cc",
     "src/base/sys-info.h",
+    "src/base/timezone-cache.h",
     "src/base/utils/random-number-generator.cc",
     "src/base/utils/random-number-generator.h",
   ]
@@ -2332,7 +2333,10 @@ v8_component("v8_libbase") {
   }
 
   if (is_posix) {
-    sources += [ "src/base/platform/platform-posix.cc" ]
+    sources += [
+      "src/base/platform/platform-posix.cc",
+      "src/base/platform/platform-posix.h",
+    ]
   }
 
   if (is_linux) {