Cache default value in StringCachedFieldTrialParameter.

When a field trial parameter doesn't exist in ChromeFeatureList,
querying ChromeFeatureList#getFieldTrialParamByFeature() will return
an empty string. This CL changes to cache the default value of this
field trial parameter when the parameter doesn't exist.

Bug: 1041865
Change-Id: I2d13a4c349e554d903e004e805dd8ce9d2f691f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072119Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744306}

chrome/browser/flags/android/java/src/org/chromium/chrome/browser/flags/StringCachedFieldTrialParameter.java

@@ -31,6 +31,7 @@ public class StringCachedFieldTrialParameter extends CachedFieldTrialParameter {
     void cacheToDisk() {
         String value =
                 ChromeFeatureList.getFieldTrialParamByFeature(getFeatureName(), getParameterName());
-        SharedPreferencesManager.getInstance().writeString(getSharedPreferenceKey(), value);
+        SharedPreferencesManager.getInstance().writeString(
+                getSharedPreferenceKey(), value.isEmpty() ? getDefaultValue() : value);
     }
 }