Add BlimpDocument, pull out functions in BlimpCompositor.

blimp/client/core/render_widget/BUILD.gn

Index: blimp/client/core/render_widget/BUILD.gn
diff --git a/blimp/client/core/render_widget/BUILD.gn b/blimp/client/core/render_widget/BUILD.gn
index 34ec7207176f22c223515d154757211777277cdd..e1c33f747998f86041e01d51c46e23bb9bb1db66 100644
--- a/blimp/client/core/render_widget/BUILD.gn
+++ b/blimp/client/core/render_widget/BUILD.gn
@@ -6,6 +6,10 @@ source_set("render_widget") {
   visibility = [ "//blimp/client/*" ]
 
   sources = [
+    "blimp_document.cc",
+    "blimp_document.h",
+    "blimp_document_manager.cc",
+    "blimp_document_manager.h",
     "render_widget_feature.cc",
     "render_widget_feature.h",
   ]
@@ -18,6 +22,7 @@ source_set("render_widget") {
 
   public_deps = [
     "//base",
+    "//blimp/client/core/compositor",
     "//blimp/net",
   ]
 }
@@ -42,6 +47,8 @@ source_set("unit_tests") {
   testonly = true
 
   sources = [
+    "blimp_document_manager_unittest.cc",
+    "blimp_document_unittest.cc",
     "render_widget_feature_unittest.cc",
   ]
 
@@ -49,11 +56,15 @@ source_set("unit_tests") {
     ":render_widget",
     ":test_support",
     "//base",
+    "//blimp/client/test/compositor",
+    "//blimp/client/test/render_widget",
     "//blimp/common",
     "//blimp/net:test_support",
     "//cc/proto",
+    "//cc/surfaces",
     "//net",
     "//net:test_support",
+    "//skia",
     "//testing/gtest",
   ]
 }