Reland of [tracing] Send unique tracing process id for cross-process memory dumps...

Reland of [tracing] Send unique tracing process id for cross-process memory dumps identification.

This is a reland of crrev.com/1173263004.
The original CL was reverted because of a static initializer. This CL
replaces the static initializer with constant 0 and the id is always
ensured to be positive and cannot be equal to invalid id.

Original message:

The memory shared between browser and renderer must be identified with
the same unique id for memory dumps in tracing. Currently the child
process is not aware of its unique child process id. But, the child
process id cannot be exposed in the child process (see discussion in
crrev.com/1155683009).

So, a hash of the child process id allocated by the browser process is
sent to child process along with begin tracing message and will be
exposed in the child process MemoryDumpManager. The allocator that
needs to create a global guid will use the id to identify memory dumps.

See the design doc https://goo.gl/ncMfUV for more details.

BUG=497726
TBR=jam@chromium.org,dcheng@chromium.org,wittman@chromium.org

Review URL: https://codereview.chromium.org/1212693003

Cr-Commit-Position: refs/heads/master@{#336376}