[Chromecast] Allow media metrics events with no session ID set
[Chromecast] Allow media metrics events with no session ID set

If a session ID is not set, we hit an assert sometimes in browser tests:

FATAL:hash_util.cc(78)] Check failed: hex.size() == 32u (0vs. 32)
#2 0x00000450354e cast::HashGUID64()
#3 0x000004503895 cast::HashSessionId64()
#4 0x0000044960d1 chromecast::CastEventBuilderImpl::SetSessionId()
#5 0x000004496530 chromecast::MetricsRecorderImpl::RecordSimpleAction()

Set a default session ID to prevent this from being reached.

BUG=

Review URL: https://codereview.chromium.org/1477483003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#362421}

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