PDF backend: don't apply paint alpha twice when we have a shader
PDF backend: don't apply paint alpha twice when we have a shader

Uncovers related bug in pdfium:
https://bugs.chromium.org/p/pdfium/issues/detail?id=1693

Bug: skia:11863
Change-Id: I764ccbfb5464f0288c66086826a1fd8bcf2ff8f7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/418544
Commit-Queue: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia