Disable WebShare on platforms other than android.

This adds origin_trial_os_whitelist to RuntimeEnabledFeatures. If set,
an origin trial will only be available on the whitelisted OSes.

BUG=635741

Review-Url: https://codereview.chromium.org/2249043002
Cr-Commit-Position: refs/heads/master@{#422981}

third_party/WebKit/Source/build/scripts/make_runtime_features.py

@@ -49,6 +49,7 @@ class RuntimeFeatureWriter(in_generator.Writer):
         'feature_policy': None,
         'implied_by': [],
         'origin_trial_feature_name': None,
+        'origin_trial_os': [],
         'settable_from_internals': False,
         'status': None,
     }

third_party/WebKit/Source/build/scripts/templates/OriginTrials.cpp.tmpl

@@ -14,8 +14,21 @@ namespace blink {
 bool OriginTrials::{{feature.first_lowered_name}}Enabled(ExecutionContext* executionContext) {
     if (RuntimeEnabledFeatures::{{feature.first_lowered_name}}Enabled())
         return true;
+{% if feature.origin_trial_os %}
+#if
+{%- for os in feature.origin_trial_os %}
+{%- if not loop.first %} ||{% endif %}
+ defined(OS_{{os | upper}})
+{%- endfor %}
+{% endif %}
+
     OriginTrialContext* context = OriginTrialContext::from(executionContext, OriginTrialContext::DontCreateIfNotExists);
     return context && context->isFeatureEnabled("{{feature.origin_trial_feature_name}}");
+{% if feature.origin_trial_os %}
+#else
+    return false;
+#endif
+{% endif %}
 }
 
 {% endif %}

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -258,7 +258,7 @@ WebFontsInterventionV2With2G
 WebFontsInterventionV2With3G
 WebFontsInterventionV2WithSlow2G
 WebFontsInterventionTrigger
-WebShare status=experimental, origin_trial_feature_name=WebShare
+WebShare status=experimental, origin_trial_feature_name=WebShare, origin_trial_os=android
 Worklet status=experimental
 ParseHTMLOnMainThread status=test
 SendBeaconThrowForBlobWithNonSimpleType status=experimental