[fuchsia] Allow official keys to remain unset in Fuchsia builds.

This is necessary because Chrome-Fuchsia packages are built and
delivered without API keys.

Bug: 1173606
Change-Id: Ibd26774f66a6224e8b6c63ee770954316980cbd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2669163
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#849817}

google_apis/google_api_keys.cc

@@ -300,10 +300,12 @@ class APIKeyCache {
     }
 
     if (key_value == DUMMY_API_TOKEN) {
-#if BUILDFLAG(GOOGLE_CHROME_BRANDING)
+#if BUILDFLAG(GOOGLE_CHROME_BRANDING) && !defined(OS_FUCHSIA)
       // No key should be unset in an official build except the
       // GOOGLE_DEFAULT_* keys.  The default keys don't trigger this
       // check as their "unset" value is not DUMMY_API_TOKEN.
+      // Exclude Fuchsia to match BUILD.gn.
+      // TODO(crbug.com/1171510): Update Fuchsia exclusion when bug is fixed.
       CHECK(false);
 #endif
       if (default_if_unset.size() > 0) {