add matcher with zero args for comparing two protos

[stevenwarejones]

This change is 

[SanjayVas]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @stevenwarejones)

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src/main/cc/common_cpp/testing/common_matchers.h line 26 at r1 (raw file):

// Returns true if two proto messages are equal, including the order of repeated
// fields.
MATCHER(EqualsProto, "") {

I'm not sure I understand the use case. The existing matcher appears to support test assertions of the form

EXPECT_THAT(actual, EqualsProto(expected))

What kind of assertion would this matcher be used for?