Revert "Enable v8 backtrace support in all debug builds"

This reverts commit 31388627.

BUG=chromium:630629

Review-Url: https://codereview.chromium.org/2171373002
Cr-Commit-Position: refs/heads/master@{#37986}

gni/v8.gni

@@ -35,7 +35,7 @@ if (v8_use_external_startup_data == "") {
 }
 
 if (v8_enable_backtrace == "") {
-  v8_enable_backtrace = is_debug
+  v8_enable_backtrace = is_debug && !v8_optimized_debug
 }
 
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