[fuchsia] Adds "fuchsia" to list of OS IDs in PolicyPrefMappingTest.

Handles the OS identifier "fuchsia" in the tests.

Bug: 1271639
Change-Id: I148ad33625df8410235726369ee03c764448af71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3403356
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962123}

components/policy/core/browser/policy_pref_mapping_test.cc

@@ -346,6 +346,8 @@ class PolicyTestCase {
     const std::string os("mac");
 #elif BUILDFLAG(IS_WIN)
     const std::string os("win");
+#elif BUILDFLAG(IS_FUCHSIA)
+    const std::string os("fuchsia");
 #else
 #error "Unknown platform"
 #endif

docs/enterprise/policy_pref_mapping_test.md

@@ -145,6 +145,7 @@ needs to have at least one test case. Valid values are:
 - `chromeos_ash`
 - `chromeos_lacros`
 - `android`
+- `fuchsia`
 - `ios` (tested via separate [policy_test_cases.json](https://cs.chromium.org/chromium/src/ios/chrome/test/data/policy/policy_test_cases.json))
 
 The boolean `official_only` field indicates whether this policy is only