[gm] Set v8_enable_fast_mksnapshot by default

This change somehow got lost when relanding an earlier CL in
https://crrev.com/c/1118139

Bug: v8:7891
Change-Id: I31244be136322cccfb465c24356f1d0fd5e0c6d6
Reviewed-on: https://chromium-review.googlesource.com/1119786Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54104}

tools/dev/gm.py

@@ -119,6 +119,7 @@ symbol_level = 2
 use_goma = {GOMA}
 goma_dir = \"{GOMA_DIR}\"
 v8_enable_backtrace = true
+v8_enable_fast_mksnapshot = true
 v8_enable_slow_dchecks = true
 v8_optimized_debug = false
 """.replace("{GOMA}", USE_GOMA).replace("{GOMA_DIR}", str(GOMADIR))
@@ -131,6 +132,7 @@ symbol_level = 1
 use_goma = {GOMA}
 goma_dir = \"{GOMA_DIR}\"
 v8_enable_backtrace = true
+v8_enable_fast_mksnapshot = true
 v8_enable_verify_heap = true
 v8_optimized_debug = true
 """.replace("{GOMA}", USE_GOMA).replace("{GOMA_DIR}", str(GOMADIR))