Adding back the byteOffset to OwnedMemory Pin

[ahsonkhan]

Supersedes #2063

Reacting to changes from dotnet/corefx#25770

cc @dotnet/corefxlab-contrib, @pakrym, @davidfowl

[ahsonkhan]

@dotnet-bot test Innerloop OSX10.12 Release Build and Test

https://ci.dot.net/job/dotnet_corefxlab/job/master/job/osx10.12_release_prtest/1428/consoleFull#-852907932d31e50d-1517-49fc-92b3-2ca637122019

[xUnit.net 00:00:25.8478100]     System.IO.Pipelines.Tests.ReadableBufferReaderFacts+OwnedMemory.CopyToLargerBufferWorks [FAIL]
18:22:55 [xUnit.net 00:00:25.8492760]       Assert.True() Failure
18:22:55 [xUnit.net 00:00:25.8494040]       Expected: True
18:22:55 [xUnit.net 00:00:25.8494680]       Actual:   False
18:22:55 [xUnit.net 00:00:25.8508200]       Stack Trace:
18:22:55 [xUnit.net 00:00:25.8672420]         /Users/dotnet-bot/j/workspace/dotnet_corefxlab/master/osx10.12_release_prtest/tests/System.Buffers.Primitives.Tests/ReadableBufferReaderFacts.cs(318,0): at System.IO.Pipelines.Tests.ReadableBufferReaderFacts.CopyToLargerBufferWorks()
18:22:55 Failed   System.IO.Pipelines.Tests.ReadableBufferReaderFacts+OwnedMemory.CopyToLargerBufferWorks
18:22:55 Error Message:
18:22:55  Assert.True() Failure
18:22:55 Expected: True
18:22:55 Actual:   False
18:22:55 Stack Trace:
18:22:55    at System.IO.Pipelines.Tests.ReadableBufferReaderFacts.CopyToLargerBufferWorks() in /Users/dotnet-bot/j/workspace/dotnet_corefxlab/master/osx10.12_release_prtest/tests/System.Buffers.Primitives.Tests/ReadableBufferReaderFacts.cs:line 318
18:23:10 [xUnit.net 00:00:41.0739380]   Finished:    System.Buffers.Primitives.Tests
18:23:11 
18:23:11 

@KrzysztofCwalina, any updates on this issue? #2062

[ahsonkhan]

Please note the removal of this check, since it was redundant with the check that Retain() already does (here and elsewhere).