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Fix sync IPC perf tests.

- Actually uses IPC::SyncChannel to send sync messages.
- More accurately record the endtime.

After this change the Mojo sync IPC is faster than IPC-over-Mojo sync IPC:
(On linux z620; release build with profiling turned on.)

MojoChannelPerfTest.ChannelProxySyncPing
IPC_ChannelProxy_Perf_50000x_144  6507.49 ms

MojoInterfacePerfTest.MultiprocessSyncPing
IPC_MultiprocessSync_Perf_50000x_144  5283.86 ms

BUG=743263

Change-Id: Ie1834869b8d88e25a5986bdc9714132d6b7fc87b
Reviewed-on: https://chromium-review.googlesource.com/575032
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487326}