[runtime] Do some more StringTable shrinking

This CL further lowers the kMaxEmptyFactor constant to more aggressively shrink
the StringTable when it's empty.

Bug: v8:5443, chromium:818642
Change-Id: I1c263a0afd7e6bed8a8bb857db032bf126c3ef4b
Reviewed-on: https://chromium-review.googlesource.com/995473Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52401}

src/objects/string-table.h

@@ -79,7 +79,7 @@ class StringTable : public HashTable<StringTable, StringTableShape> {
 
   DECL_CAST(StringTable)
 
-  static const int kMaxEmptyFactor = 8;
+  static const int kMaxEmptyFactor = 4;
   static const int kMinCapacity = 2048;
   static const int kMinShrinkCapacity = kMinCapacity;