Shrink PLSA header by 40%.

This patch reduces preprocessed size of paint_layer_scrollable_area.h
from 6.3 MB to 3.8 MB, avoiding transitive inclusion of chrome_client.h
and float_clip_rect.h with tricks similar to crrev.com/567547.  (Since
crrev.com/567732, more source files include PLSA's header.)

Bug: 242216,823365
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I969088b6f75b80d3279b0beb3cc701eb16e6ed6a
Reviewed-on: https://chromium-review.googlesource.com/1103235
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567868}
5 files changed