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Defines a base::debug::WriteClangCoverageProfile() API which performs
an explicit call to __llvm_profile_dump() in CLANG_COVERAGE builds, with
locking to ensure thread-safety.

This API is invoked in the base::debug::BreakDebugger() and
base::Process::TerminateCurrentProcessImmediately() APIs, immediately
prior to them terminating the process, to mirror the coverage-writing
step that would be performed via an at-exit handler during a normal
process-exit.

This ensures that we get as complete coverage data as possible from
processes which fast-terminate/break (e.g. EXPECT_DEATH() sub-processes,
e.g. browser child processes, etc).

Bug: 849369
Change-Id: I38262334bb5abf8d5ba40c2c32352b38096905ec
Reviewed-on: https://chromium-review.googlesource.com/1172932Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583882}