build: Default optimize_for_size to true for Fuchsia

Fuchsia cares about binary size, like iOS and Android. This configures
the default build to -Oz instead of -O2.  This reduces the size of text
segment of the service_exe webrunner executable by about 13% when
building for ARM64 release mode non-component:

O2:
   text    data     bss     dec     hex filename
83509089        3370852 1109788a 87989729        53e9de1 out/fuchsia_arm64/exe.unstripped/service_exe

   text    data     bss     dec     hex filename
73707333        3398764 1110516 78216613        4a97da5 out/fuchsia_arm64/exe.unstripped/service_exe

Oz: 
Change-Id: I09774eb71f690a483f3f6064cace265a7f7f80e6
Reviewed-on: https://chromium-review.googlesource.com/1171864Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582460}

build/config/compiler/compiler.gni

@@ -83,7 +83,7 @@ exclude_unwind_tables =
 # Linux & Mac favor speed over size.
 # TODO(brettw) it's weird that Mac and desktop Linux are different. We should
 # explore favoring size over speed in this case as well.
-optimize_for_size = is_android || is_ios
+optimize_for_size = is_android || is_ios || is_fuchsia
 
 declare_args() {
   # Whether we should consider the profile we're using to be accurate. Accurate