[NTP Snippets] 📷 Add placeholders and start caching thumbnails

chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml

Index: chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml
diff --git a/chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml b/chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml
new file mode 100644
index 0000000000000000000000000000000000000000..52b085d5d6a055018eeb6694e8fe20acdc28a30a
--- /dev/null
+++ b/chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml
@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- TODO(https://crbug.com/602627) tools:targetApi added to mute the NewApi lint error. -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    tools:targetApi="21"
+    android:width="@dimen/snippets_thumbnail_size"
+    android:height="@dimen/snippets_thumbnail_size"
+    android:viewportWidth="72"
+    android:viewportHeight="72">
+
+    <path android:pathData="M0,0 L72,0 L72,71.9997 L0,71.9997 Z" />
+    <path android:fillColor="@color/snippets_thumnail_placeholder_fg"
+        android:pathData="M45,42.9997 L45,28.9997 C45,27.8997,44.1,26.9997,43,26.9997 L29,26.9997
+C27.9,26.9997,27,27.8997,27,28.9997 L27,42.9997
+C27,44.0997,27.9,44.9997,29,44.9997 L43,44.9997
+C44.1,44.9997,45,44.0997,45,42.9997 Z M32.5,37.4997 L35,40.5097 L38.5,35.9997
+L43,41.9997 L29,41.9997 L32.5,37.4997 Z" />
+</vector>