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

tools/telemetry/telemetry/core/tab.py

Index: tools/telemetry/telemetry/core/tab.py
diff --git a/tools/telemetry/telemetry/core/tab.py b/tools/telemetry/telemetry/core/tab.py
index a9776573c360d542ec7d86ff88c6fb9e7a4f4821..eb7c1083da134c8858c9fbd0a19be61bf76759c0 100644
--- a/tools/telemetry/telemetry/core/tab.py
+++ b/tools/telemetry/telemetry/core/tab.py
@@ -71,7 +71,7 @@ class Tab(web_contents.WebContents):
     """Capture a screenshot of the tab's contents.
 
     Returns:
-      A telemetry.core.backends.png_bitmap.
+      A telemetry.core.Bitmap.
     """
     return self._inspector_backend.Screenshot(timeout)
 
@@ -97,7 +97,7 @@ class Tab(web_contents.WebContents):
       (time_ms, bitmap) tuples representing each video keyframe. Only the first
       frame in a run of sequential duplicate bitmaps is included.
         time_ms is milliseconds since navigationStart.
-        bitmap is a telemetry.core.backends.png_bitmap.
+        bitmap is a telemetry.core.Bitmap.
     """
     # TODO(tonyg/szym): platform's video capture may include offscreen content
     # from a webcam, OS framing and browser framing. However, this API must