Add blob scheme in Navigation Histograms

Added 'blob' scheme in Navigation.MainFrameScheme and Navigation.MainFrameSchemeDifferentPage histograms.
Added it in the end, before the scheme_max so that scheme_max contains the number of schemes.

BUG=492773

Review URL: https://codereview.chromium.org/1167403002

Cr-Commit-Position: refs/heads/master@{#333665}

components/navigation_metrics/navigation_metrics.cc

@@ -9,6 +9,8 @@
 
 namespace {
 
+// This enum is used in building the histogram. So, this is append only,
+// any new scheme should be added at the end, before SCHEME_MAX
 enum Scheme {
   SCHEME_UNKNOWN,
   SCHEME_HTTP,
@@ -19,6 +21,7 @@ enum Scheme {
   SCHEME_JAVASCRIPT,
   SCHEME_ABOUT,
   SCHEME_CHROME,
+  SCHEME_BLOB,
   SCHEME_MAX,
 };
 
@@ -32,6 +35,7 @@ const char* const kSchemeNames[] = {
   url::kJavaScriptScheme,
   url::kAboutScheme,
   "chrome",
+  url::kBlobScheme,
   "max",
 };
 

tools/metrics/histograms/histograms.xml

@@ -60217,6 +60217,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="6" label="javascript"/>
   <int value="7" label="about"/>
   <int value="8" label="chrome"/>
+  <int value="9" label="blob"/>
 </enum>
 
 <enum name="NetCacheState" type="int">