Use CastSysInfoDummy for fuchsia

This is a temporary solution until we implement a Fuchsia version of
CastSysInfo. The only downside to using the dummy is that the strings,
such as serial number, model, build, etc are all faked out.

Bug: internal b/126768956
Test: CQ
Change-Id: I3364d87c57d7f40dbe2d1e80b19bb2f02532512b
Reviewed-on: https://chromium-review.googlesource.com/c/1496419Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Auto-Submit: Aidan Wolter <awolter@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636644}

chromecast/base/BUILD.gn

@@ -228,7 +228,7 @@ cast_source_set("cast_sys_info") {
       "//chromecast:chromecast_buildflags",
       "//chromecast/browser:jni_headers",
     ]
-  } else if (chromecast_branding == "public") {
+  } else if (chromecast_branding == "public" || is_fuchsia) {
     sources += [ "cast_sys_info_util_simple.cc" ]
   }
 }
@@ -249,7 +249,7 @@ cast_shared_library("libcast_sys_info_1.0") {
 
 # Note: Android links //chromecast/base:cast_sys_info statically.
 cast_source_set("cast_sys_info_shlib") {
-  if (!is_android) {
+  if (!is_android && !is_fuchsia) {
     sources = [
       "cast_sys_info_util_shlib.cc",
     ]