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This adds trace entries for loading imports and for blocking on an
import for longer than 20ms. These make gn's tracelog significantly
easier to understand when imported files take a long time to process.
For example, in the Chrome build on Mac //build/toolchain/mac/BUILD.gn
blocks for ~240ms running sdk_info.py because
//build/config/mac/mac_sdk.gni runs that script and //ui/base/BUILD.gn
blocks for ~350ms because it is waiting for the import lock on
mac_sdk.gni to be released. Without these traces, it is not
immediately obvious why //build/toolchain/mac/BUILD.gn is running that
script and it is very unclear why //ui/base/BUILD.gn takes so long.

Review-Url: https://codereview.chromium.org/2424233002
Cr-Commit-Position: refs/heads/master@{#427151}