[runtime] Decrease StringTable shrink limit

Given that we have not seen any regressions yet we're trying a more aggressive
limit.

Bug: chromium:818642, v8:5443
Change-Id: Ic45001ed6c042fc31cbba0d417d5060d2de8fb3a
Reviewed-on: https://chromium-review.googlesource.com/975126Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52145}

src/objects/string-table.h

@@ -78,7 +78,7 @@ class StringTable : public HashTable<StringTable, StringTableShape> {
 
   DECL_CAST(StringTable)
 
-  static const int kMaxEmptyFactor = 16;
+  static const int kMaxEmptyFactor = 8;
   static const int kMinCapacity = 2048;
   static const int kMinShrinkCapacity = kMinCapacity;