[snapshot] Ensure padding is always cleared in tests

Re-enables and fixes msan test failures due to string padding being
cleared only selectively in tests. This change instead makes sure it
always happens in TestIsolate.

Bug: v8:7746
Change-Id: I259b43ad25cb7af18bf16d29effb15772c981a67
Reviewed-on: https://chromium-review.googlesource.com/1051647Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53095}

test/cctest/cctest.status

@@ -198,11 +198,6 @@
 ['msan == True', {
   # ICU upstream issues.
   'test-strings/CountBreakIterator': [SKIP],
-
-  # https://crbug.com/v8/7746
-  # MSan issues after landing https://crrev.com/c/1046657
-  'test-serialize/StartupSerializerOnce': [SKIP],
-  'test-serialize/StartupSerializerTwice': [SKIP],
 }],  # 'msan == True'
 
 ##############################################################################

test/cctest/test-serialize.cc

@@ -81,6 +81,7 @@ class TestIsolate : public Isolate {
     v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate);
     v8::Isolate::Scope isolate_scope(v8_isolate);
     isolate->Init(nullptr);
+    isolate->heap()->read_only_space()->ClearStringPaddingIfNeeded();
     return v8_isolate;
   }
   // Wraps v8::Isolate::New, but with a TestIsolate under the hood.
@@ -458,7 +459,6 @@ static void PartiallySerializeContext(Vector<const byte>* startup_blob_out,
 
     env.Reset();
 
-    isolate->heap()->read_only_space()->ClearStringPaddingIfNeeded();
     SnapshotByteSink startup_sink;
     StartupSerializer startup_serializer(isolate);
     startup_serializer.SerializeStrongReferences();
@@ -583,7 +583,6 @@ static void PartiallySerializeCustomContext(
 
     env.Reset();
 
-    isolate->heap()->read_only_space()->ClearStringPaddingIfNeeded();
     SnapshotByteSink startup_sink;
     StartupSerializer startup_serializer(isolate);
     startup_serializer.SerializeStrongReferences();