[Telemetry] Rename PngBitmap->Bitmap and move it to core.

content/test/gpu/gpu_tests/maps.py

Index: content/test/gpu/gpu_tests/maps.py
diff --git a/content/test/gpu/gpu_tests/maps.py b/content/test/gpu/gpu_tests/maps.py
index 4727cfd8362264ffc35e18c6e797b04b23b54642..2224120822c10e483e1cbdb2d4d3f63c5a47039b 100644
--- a/content/test/gpu/gpu_tests/maps.py
+++ b/content/test/gpu/gpu_tests/maps.py
@@ -14,7 +14,7 @@ import re
 import maps_expectations
 
 from telemetry import test
-from telemetry.core.backends import png_bitmap
+from telemetry.core.backends import bitmap
 from telemetry.core import util
 from telemetry.page import page_test
 from telemetry.page import page_set
@@ -97,7 +97,7 @@ class MapsValidator(page_test.PageTest):
           (x, y, screenshot.width, screenshot.height))
 
       pixel_color = screenshot.GetPixelColor(x, y)
-      expect_color = png_bitmap.PngColor(
+      expect_color = bitmap.RgbaColor(
           expectation["color"][0],
           expectation["color"][1],
           expectation["color"][2])