DPWA: update fieldtrial param name for install In-Product Help

Rename field trial param for site engagement threshold to have a "x_"
suffix so that it won't be parsed by IPH feature engagement tracker.
fieldtrial params are parsed by feature_engagement::Tracker to apply
configurations generic to IPH framework. For PWA install IPH we implemented
custom logic on top of IPH framework to only show IPH when site
engagement score is above X. According to IPH doc https://chromium.googlesource.com/chromium/src/+/master/components/feature_engagement/README.md#featureconfig
if a non IPH predefined field is specified without "x_" it will
be recorded as FAILURE_UNKNOWN_KEY in the
InProductHelp.Config.ParsingEvent histogram.

Bug: 1150502
Change-Id: Id72ccfaa7855052ed8ad3010ae1c6c87ef5cf7ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545261
Commit-Queue: Phillis Tang <phillis@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828879}

chrome/browser/ui/views/page_action/pwa_install_view.cc

@@ -44,9 +44,10 @@ constexpr base::FeatureParam<ExperimentIcon> kInstallIconParam{
     &kIconParamOptions};
 
 // Site engagement score threshold to show In-Product Help.
+// Add x_ prefix so the IPH feature engagement tracker can ignore this.
 constexpr base::FeatureParam<int> kIphSiteEngagementThresholdParam{
     &feature_engagement::kIPHDesktopPwaInstallFeature,
-    "siteEngagementThreshold", 10};
+    "x_site_engagement_threshold", 10};
 
 }  // namespace