Reduce VC++ padding in IncrementalMarking class

IncrementalMarking has nine bytes of padding in 32-bit and 64-bit
builds. Fixing 32-bit builds just requires moving the one-byte
incremental_marking_job_ member. Fixing 64-bit requires moving the
four-byte state_ member. This change reduces the padding to one byte.

On 64-bit this reduces its size from 152 to 144. This also fits heap
granularity better. On 32-bit it goes from 96 to 88 bytes.

The initial padding was found with llvm-pdbdump.exe. The fix was
verified by compiling v8/src/assembler.cc with the undocumented
/d1reportSingleClassLayout option, like this:

    /d1reportSingleClassLayoutIncrementalMarking

The savings should apply on all platforms, or at worst should make no
difference except for improving alignment.

Thanks to zturner@ for some llvm-pdbdump improvements.

BUG=chromium:710933

Review-Url: https://codereview.chromium.org/2808473003
Cr-Commit-Position: refs/heads/master@{#44698}

src/heap/incremental-marking.cc

@@ -23,10 +23,10 @@ namespace internal {
 
 IncrementalMarking::IncrementalMarking(Heap* heap)
     : heap_(heap),
-      state_(STOPPED),
       initial_old_generation_size_(0),
       bytes_marked_ahead_of_schedule_(0),
       unscanned_bytes_of_large_object_(0),
+      state_(STOPPED),
       idle_marking_delay_counter_(0),
       incremental_marking_finalization_rounds_(0),
       is_compacting_(false),

src/heap/incremental-marking.h

@@ -271,8 +271,6 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
 
   Heap* heap_;
 
-  State state_;
-
   double start_time_ms_;
   size_t initial_old_generation_size_;
   size_t old_generation_allocation_counter_;
@@ -280,6 +278,8 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   size_t bytes_marked_ahead_of_schedule_;
   size_t unscanned_bytes_of_large_object_;
 
+  State state_;
+
   int idle_marking_delay_counter_;
   int incremental_marking_finalization_rounds_;
 
@@ -289,10 +289,10 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   bool black_allocation_;
   bool finalize_marking_completed_;
   bool trace_wrappers_toggle_;
+  IncrementalMarkingJob incremental_marking_job_;
 
   GCRequestType request_type_;
 
-  IncrementalMarkingJob incremental_marking_job_;
   Observer new_generation_observer_;
   Observer old_generation_observer_;