[vm] Avoid data race during Dart_IsolateGroupHeapNewUsedMetric.

Precise new-space usage requires looking at all the TLAB pointers and normally only occurs under a GC safepoint. For external queries coarsen the answer to capacity.

TEST=g3
Bug: b/324221135
Change-Id: I0e83bbcddba21ddf539fb2afdedf2959cef2c078
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352682
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

https://dart.googlesource.com/sdk/+/a4072803b3de9eae5960411898d480e64c69a030

DEPS

diff --git a/DEPS b/DEPS
index 78f5037..3b342c7 100644
--- a/DEPS
+++ b/DEPS
@@ -279,7 +279,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '76193cdce2f60b35022d975809db692256523966',
 
-  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '974dfa30713211634f0054b47ddadce02e7d7d4b',
+  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a4072803b3de9eae5960411898d480e64c69a030',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '0fac13b4437cbd3d2f5ac1a576b4aadb40aa95c8',
 

commits.json

diff --git a/commits.json b/commits.json
index b972364..3af55ba 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"76193cdce2f60b35022d975809db692256523966",
-  "engine/src/third_party/dart":"974dfa30713211634f0054b47ddadce02e7d7d4b",
+  "engine/src/third_party/dart":"a4072803b3de9eae5960411898d480e64c69a030",
   "flutter":"0fac13b4437cbd3d2f5ac1a576b4aadb40aa95c8"
 }
\ No newline at end of file