Remove optimization to lazily allocate transparency layers in LayerPainter.

Before this CL, LayerPainter would try to avoid starting a transparency layer
until it encountered actual drawable content within the RenderLayer that has transparency, or its child.

This optimization makes for code complexity and speed issues, makes it hard to
implement clean display lists, and is most likely not a useful optimization.
It is likely not useful for two reasons:

* It only has effect if the webpage has opacity but no actual content. That does not seem common.
* Skia already has optimizations to remove unnecessary layers if there is no content within them:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/src/core/SkRecordOpts.cpp&q=SkRecordNoopSaveRestores&sq=package:chromium&type=cs&l=21

BUG=432755

Review URL: https://codereview.chromium.org/725433002

git-svn-id: svn://svn.chromium.org/blink/trunk@185324 bbb929c8-8fbe-4397-9dbb-9b2b20218538