Disable debug fission under Fuchsia.

The symbolization in the Fuchsia runner scripts is not set up to work
with debug information partly split from the binary.

Bug: 777008
Change-Id: Iedaf4fe56cc8092af21acfd0d244304982a6035c
Reviewed-on: https://chromium-review.googlesource.com/1170440Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582344}

build/config/compiler/BUILD.gn

@@ -168,7 +168,7 @@ if (gold_path == false) {
 }
 
 if (use_debug_fission == "default") {
-  use_debug_fission = is_debug && !is_android && !is_win &&
+  use_debug_fission = is_debug && !is_android && !is_fuchsia && !is_win &&
                       (use_gold || use_lld) && cc_wrapper == ""
 }