Fuchsia: implement OS::Allocate

Fix v8 link errors (in component_build=true)
https://build.chromium.org/p/chromium.fyi/builders/Fuchsia%20(dbg)

BUG=chromium:731217

Review-Url: https://codereview.chromium.org/2930343002
Cr-Commit-Position: refs/heads/master@{#45878}

src/base/platform/platform-fuchsia.cc

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <sys/mman.h>
+
 #include "src/base/macros.h"
 #include "src/base/platform/platform-posix.h"
 #include "src/base/platform/platform.h"
@@ -11,6 +13,17 @@ namespace base {
 
 TimezoneCache* OS::CreateTimezoneCache() { return new PosixTimezoneCache(); }
 
+void* OS::Allocate(const size_t requested, size_t* allocated,
+                   OS::MemoryPermission access) {
+  const size_t msize = RoundUp(requested, AllocateAlignment());
+  int prot = GetProtectionFromMemoryPermission(access);
+  void* addr = OS::GetRandomMmapAddr();
+  void* mbase = mmap(addr, msize, prot, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (mbase == MAP_FAILED) return NULL;
+  *allocated = msize;
+  return mbase;
+}
+
 std::vector<OS::SharedLibraryAddress> OS::GetSharedLibraryAddresses() {
   CHECK(false);  // TODO(fuchsia): Port, https://crbug.com/731217.
   return std::vector<SharedLibraryAddress>();