[build] Disable slow asserts for v8_enable_fast_mksnapshot

Slow asserts double the time spent in mksnapshot. While we want them
enabled on our bot builds, local builds should be fast and don't
necessarily need them.

This also adds v8_enable_fast_mksnapshot as default in gm.py.

Bug: v8:7891,v8:6688
Change-Id: I5ab1491f3396c4351de30dbbb80a81e2242c5db9
Reviewed-on: https://chromium-review.googlesource.com/1116552Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54074}

BUILD.gn

@@ -1059,6 +1059,7 @@ template("run_mksnapshot") {
 
     if (v8_enable_fast_mksnapshot) {
       args += [
+        "--no-enable-slow-asserts",
         "--no-turbo-rewrite-far-jumps",
         "--no-turbo-verify-allocation",
       ]

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))