Revert "Resize identity map by doubling instead of quadrupling."

This reverts commit 3d82e557.

Reason for revert: gc stress failures, e.g. https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/9729/steps/Mjsunit%20%28flakes%29/logs/regress-crbug-647217

Original change's description:
> 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}

TBR=rmcilroy@chromium.org,titzer@chromium.org,jochen@chromium.org,v8-reviews@googlegroups.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Change-Id: I96cc0e2ca225b9a76eaaf604cd240e71176736ba
Reviewed-on: https://chromium-review.googlesource.com/459513Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44146}

src/identity-map.cc

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