Split GrResourceAllocator::assign into two methods

A follow-up CL allows the user to check the budget headroom
in between these two steps.

Bug: skia:10877
Change-Id: Ie0e6f23029065158b2eadb0dc26caf32f54433e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/394999
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrDrawingManager.cpp b/src/gpu/GrDrawingManager.cpp
index 512911a..9f983f6 100644
--- a/src/gpu/GrDrawingManager.cpp
+++ b/src/gpu/GrDrawingManager.cpp
@@ -211,7 +211,7 @@
             task->gatherProxyIntervals(&alloc);
         }
 
-        flushed = alloc.assign() && this->executeRenderTasks(&flushState);
+        flushed = alloc.planAssignment() && alloc.assign() && this->executeRenderTasks(&flushState);
     }
     this->removeRenderTasks();