[heap] Set number of fixpoint iterations to 10

Set number of fixpoint iterations to 10 before switching to the linear
algorithm. The flag's initial default value was set to 0 to use the
linear algorithm all the time for testing purposes.

Bug: chromium:844008
Change-Id: Ib0fd6241b55468ef5be2c4cc3cc147db2d432c83
Reviewed-on: https://chromium-review.googlesource.com/1134769Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#54413}

src/flag-definitions.h

@@ -704,7 +704,7 @@ DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING_BOOL,
             "use concurrent marking")
 DEFINE_BOOL(parallel_marking, true, "use parallel marking in atomic pause")
 DEFINE_IMPLICATION(parallel_marking, concurrent_marking)
-DEFINE_INT(ephemeron_fixpoint_iterations, 0,
+DEFINE_INT(ephemeron_fixpoint_iterations, 10,
            "number of fixpoint iterations it takes to switch to linear "
            "ephemeron algorithm")
 DEFINE_BOOL(trace_concurrent_marking, false, "trace concurrent marking")