OOP PDF - Allow OOP PDF to use histograms

This patch adds libpdf.so to whitelisted plugins so that
it can use histograms. Without this patch, an error is
displayed every time when Chrome is launched with OOP PDF flag
and PDF file is opened.

BUG=405305

Review URL: https://codereview.chromium.org/508543002

Cr-Commit-Position: refs/heads/master@{#292109}

chrome/renderer/pepper/pepper_uma_host.cc

@@ -33,7 +33,8 @@ const char* const kWhitelistedHistogramPrefixes[] = {
 };
 
 const char* const kWhitelistedPluginBaseNames[] = {
-    "libwidevinecdmadapter.so"  // see http://crbug.com/368743
+    "libwidevinecdmadapter.so",  // see http://crbug.com/368743
+    "libpdf.so"                  // see http://crbug.com/405305
 };
 
 std::string HashPrefix(const std::string& histogram) {