[PartitionAlloc] Enable GigaCage by default

No performance / memory / stability regression is observed. See comments
#34 and #35 on crbug/1086388


Bug: 1086388, 1093838
Change-Id: I367b781a681b350883cde39b27ac505b3722bbba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548702
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829108}

base/allocator/partition_allocator/partition_alloc_features.cc

@@ -13,11 +13,11 @@ namespace features {
 // If enabled, PartitionAllocator reserves an address space(named, giga cage)
 // initially and uses a part of the address space for each allocation.
 const Feature kPartitionAllocGigaCage{"PartitionAllocGigaCage",
-                                      FEATURE_DISABLED_BY_DEFAULT};
+                                      FEATURE_ENABLED_BY_DEFAULT};
 #else
 // If enabled, PartitionAllocator remembers allocated address space.
 const Feature kPartitionAllocGigaCage{"PartitionAllocGigaCage32bit",
-                                      FEATURE_DISABLED_BY_DEFAULT};
+                                      FEATURE_ENABLED_BY_DEFAULT};
 #endif
 
 // If enabled, PCScan is turned on by default for all partitions that don't