Avoid excessive GCs in small heaps.

Small heaps and small heap growing factor can lead to excessive GCs in corner cases.

Consider function F(old_gen_size, factor) that returns the number of bytes that
have to be allocated in the old generation to start incremental marking.

F(4MB, 1.1) = 4MB (because of kMinimumOldGenerationAllocationLimit)
F(6MB, 1.1) = 2MB (because of kMinimumOldGenerationAllocationLimit)
F(8MB, 1.1) = 800KB

Funtion F should be monotonic in old_gen_size, but it currently has a minimum
at kMinimumOldGenerationAllocationLimit.

This CL makes F monotonic.

BUG=

Review URL: https://codereview.chromium.org/1144223002

Cr-Commit-Position: refs/heads/master@{#28549}

src/heap/heap.cc

@@ -5303,7 +5303,7 @@ intptr_t Heap::CalculateOldGenerationAllocationLimit(double factor,
   CHECK(factor > 1.0);
   CHECK(old_gen_size > 0);
   intptr_t limit = static_cast<intptr_t>(old_gen_size * factor);
-  limit = Max(limit, kMinimumOldGenerationAllocationLimit);
+  limit = Max(limit, old_gen_size + kMinimumOldGenerationAllocationLimit);
   limit += new_space_.Capacity();
   intptr_t halfway_to_the_max = (old_gen_size + max_old_generation_size_) / 2;
   return Min(limit, halfway_to_the_max);