The spec change is: https://github.com/w3c/webcomponents/commit/9b7f16e90b88594ed783ff7e0f971fad084e5f4d

Blink should follow the change of the terminologies in the spec.
There is no change on web-facing APIs.

This CL is the result of the (almost) mechanical renaming, using:

> zargs **/*.{h,cpp,gyp,gypi,idl,js,html,txt} -- sed -i -e 's/ComposedTree/FlatTree/g'
> zargs **/*.{h,cpp,gyp,gypi,idl,js,html,txt} -- sed -i -e 's/composed tree/flat tree/g'
> zargs **/*.{h,cpp,gyp,gypi,idl,js,html,txt} -- sed -i -e 's/Composed tree/Flat tree/g'
> zargs **/*.{h,cpp,gyp,gypi,idl,js,html,txt} -- sed -i -e 's/Composed Tree/Flat Tree/g'

Other renaming, which would require a more careful investigation, will be done in another CL.

BUG=531990

Review URL: https://codereview.chromium.org/1675163002

Cr-Commit-Position: refs/heads/master@{#374095}