[Paint Preview] Fix white rect in the bottom in player

On Android M and prior versions, a white rect appears on the bottom of
the player. This is seemingly caused by using the canvas#setMatrix
method.

This CL removes this call for identity matrix which fixes this problem
for the default scale factor.
However, for non-identity matrices, the white rect will still appear.
Fixing that requires further investigation.

Bug: 1126541
Change-Id: I9c48b3dc90e6fecf450f99059ff8cdf788d94420
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401840Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805428}

components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/frame/PlayerFrameView.java

@@ -99,7 +99,7 @@ class PlayerFrameView extends FrameLayout {
     @Override
     protected void onDraw(Canvas canvas) {
         canvas.save();
-        canvas.setMatrix(mScaleMatrix);
+        if (!mScaleMatrix.isIdentity()) canvas.setMatrix(mScaleMatrix);
         mBitmapPainter.onDraw(canvas);
         canvas.restore();
     }