Resize identity map by doubling instead of quadrupling.

Perf-sheriffs please revert if this causes any performance regressions.

BUG=

Change-Id: I9ead7148e1e0146ece3614df49fd031dd1e357fe
Reviewed-on: https://chromium-review.googlesource.com/445159
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44143}

src/identity-map.cc

@@ -11,7 +11,7 @@ namespace v8 {
 namespace internal {
 
 static const int kInitialIdentityMapSize = 4;
-static const int kResizeFactor = 4;
+static const int kResizeFactor = 2;
 
 IdentityMapBase::~IdentityMapBase() {
   // Clear must be called by the subclass to avoid calling the virtual
@@ -86,7 +86,8 @@ void* IdentityMapBase::DeleteIndex(int index) {
   size_--;
   DCHECK_GE(size_, 0);
 
-  if (size_ * kResizeFactor < capacity_ / kResizeFactor) {
+  if (capacity_ > kInitialIdentityMapSize &&
+      size_ * kResizeFactor < capacity_ / kResizeFactor) {
     Resize(capacity_ / kResizeFactor);
     return ret_value;  // No need to fix collisions as resize reinserts keys.
   }