Add flag to enable Material Design for the PDF Viewer.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 24890235bf659df419fc56dfc6d3ab4c177587ef..c6ea6285b8b9c7a494ec514621c4336b7729a3cb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -50809,6 +50809,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-1319688939" label="ignore-gpu-blacklist"/>
   <int value="-1285021473" label="save-page-as-mhtml"/>
   <int value="-1268836676" label="disable-out-of-process-pdf"/>
+  <int value="-1267958145" label="disable-pdf-material-ui"/>
   <int value="-1241747717" label="enable-android-password-link"/>
   <int value="-1218608640" label="disable-offline-load-stale-cache"/>
   <int value="-1212273428" label="enable-experimental-app-list"/>
@@ -50857,6 +50858,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-660160292" label="enable-apps-show-on-first-paint"/>
   <int value="-649956990" label="enable-harfbuzz-rendertext"/>
   <int value="-641719457" label="disable-compositor-touch-hit-testing"/>
+  <int value="-622685174" label="enable-pdf-material-ui"/>
   <int value="-604814313" label="enable-pinch"/>
   <int value="-601384286" label="disable-contextual-search"/>
   <int value="-579192400" label="disable-input-view"/>