Port v8_optimized_debug to gn.

Makes browser_tests --gtest_filter=CrSettingsBrowserTest.CrSettingsTest
complete in a little over 8 seconds instead of timing out after 45s
on my box.  Without this patch, the test is much slower in gn builds
than in gyp builds.

src/ part, goes with https://codereview.chromium.org/1882453002

BUG=586511

Review URL: https://codereview.chromium.org/1880693003

Cr-Commit-Position: refs/heads/master@{#386524}

build_overrides/v8.gni

@@ -14,6 +14,9 @@ enable_correct_v8_arch = false
 # iOS - http://crbug.com/545676
 v8_use_external_startup_data = !is_ios
 
+# Turns on compiler optimizations in V8 in Debug build.
+v8_optimized_debug = true
+
 # V8 extras
 # Adding V8 extras files requires API owners review
 # Be sure to synchronize with build/common.gypi