Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/os_android.cc

Index: runtime/vm/os_android.cc
diff --git a/runtime/vm/os_android.cc b/runtime/vm/os_android.cc
index 66063814002d742fa1d445aea0c0c654b4d6049d..676e2780a7058f5fa7b1559a1c7b11ba1dc02ec4 100644
--- a/runtime/vm/os_android.cc
+++ b/runtime/vm/os_android.cc
@@ -76,7 +76,7 @@ class PerfCodeObserver : public CodeObserver {
     const char* format = "%" Px " %" Px " %s%s\n";
     const char* marker = optimized ? "*" : "";
     intptr_t len = OS::SNPrint(NULL, 0, format, base, size, marker, name);
-    char* buffer = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
+    char* buffer = Thread::Current()->zone()->Alloc<char>(len + 1);
     OS::SNPrint(buffer, len + 1, format, base, size, marker, name);
     (*file_write)(buffer, len, out_file_);
   }
@@ -109,7 +109,7 @@ class GdbCodeObserver : public CodeObserver {
       // <name> for rest of the code (first instruction is prologue sequence).
       const char* kFormat = "%s_%s";
       intptr_t len = OS::SNPrint(NULL, 0, kFormat, name, "entry");
-      char* pname = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
+      char* pname = Thread::Current()->zone()->Alloc<char>(len + 1);
       OS::SNPrint(pname, (len + 1), kFormat, name, "entry");
       DebugInfo::RegisterSection(pname, base, size);
       DebugInfo::RegisterSection(name,