[oilpan] Enable incremental marking buildflag

Includes incremental garbage collection infrastructure in regular
builds. Does not enable incremental marking at runtime!

This CL may cause throughput regressions and is an attempt
to collect a broad range of potential performance issues.

Bug: chromium:757440
Change-Id: If4d91be2eb325417736a38e27c481ab23ce7a0e9
Reviewed-on: https://chromium-review.googlesource.com/1041986Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555698}

third_party/blink/renderer/platform/heap/BUILD.gn

@@ -8,16 +8,14 @@ import("//third_party/blink/renderer/platform/platform.gni")
 import("//testing/test.gni")
 
 declare_args() {
-  # Enables incremental marking in Oilpan.
+  # Build Blink with incremental marking infrastructure for Oilpan.
   #
-  # Note: Incremental marking is currently considered experimental and also
-  # enables 'enable_blink_heap_incremental_marking'. See default value below.
-  enable_blink_heap_incremental_marking = false
-}
+  # To turn on incremental marking also use
+  #   --enable-blink-features=HeapIncrementalMarking
+  enable_blink_heap_incremental_marking = true
 
-declare_args() {
   # Enables heap verification.
-  enable_blink_heap_verification = enable_blink_heap_incremental_marking
+  enable_blink_heap_verification = false
 }
 
 buildflag_header("blink_heap_buildflags") {