Add non-inline SkRTreeFactory constructor to fix clang-cl

Add non-inline SkRTreeFactory constructor to fix clang-cl

When Chromium is switched to build with VS 2015 that switches clang-cl
to build in -fmsc-version=1900 mode. This causes some changes in
importing of constructors and vftables which leads to link errors in
blink_platform.dll. Adding a non-inline constructor makes 1900 mode
behave consistently with 1800 mode and avoids the link error.

The root cause needs investigation but this will unblock the VS 2015
switch.

BUG=440500,498544
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Review URL: https://codereview.chromium.org/1676833002