store info in basedevice, change getter to non-virtual const&

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2261003003

Review-Url: https://codereview.chromium.org/2261003003
diff --git a/src/pdf/SkPDFDevice.cpp b/src/pdf/SkPDFDevice.cpp
index edf1728..73d41b1 100644
--- a/src/pdf/SkPDFDevice.cpp
+++ b/src/pdf/SkPDFDevice.cpp
@@ -559,7 +559,8 @@
 ////////////////////////////////////////////////////////////////////////////////
 
 SkPDFDevice::SkPDFDevice(SkISize pageSize, SkScalar rasterDpi, SkPDFDocument* doc, bool flip)
-    : INHERITED(SkSurfaceProps(0, kUnknown_SkPixelGeometry))
+    : INHERITED(SkImageInfo::MakeUnknown(pageSize.width(), pageSize.height()),
+                SkSurfaceProps(0, kUnknown_SkPixelGeometry))
     , fPageSize(pageSize)
     , fContentSize(pageSize)
     , fExistingClipRegion(SkIRect::MakeSize(pageSize))
@@ -1294,11 +1295,6 @@
                                 &content.entry()->fContent);
 }
 
-SkImageInfo SkPDFDevice::imageInfo() const {
-    SkImageInfo info = SkImageInfo::MakeUnknown(fPageSize.width(), fPageSize.height());
-    return info;
-}
-
 sk_sp<SkSurface> SkPDFDevice::makeSurface(const SkImageInfo& info, const SkSurfaceProps& props) {
     return SkSurface::MakeRaster(info, &props);
 }