GlaxoSmithKline plc (LSE/NYSE:GSK) has reached a deal to acquire Sierra Oncology (Nasdaq:SRRA) to bolster its oncology pipeline. Under the terms of the proposed deal, GSK would pay $44 per share for a total value of approximately $1.9 billion. GSK shares were mostly flat in mid-day trading, dipping 0.032% to $46.52. SRRA shares surged 38.31% to…