[Contextual Search] Robustness for SendSurroundingText.

[Contextual Search] Robustness for SendSurroundingText.

Update code calling SendSurroundingText to make sure all
values are reasonable to avoid crashes reported in these
bugs.  I do not have a reproducable case to manually
recreate the crash, so just making the code more
robust, with the aim that this will prevent these
crashes.  My guess at the failure is a negative
start offset being returned by the surrounding text
Blink callback.

Also made two changes to make the code more readable:
1) Changed usage of base::string16#size to use
base::string16#length to get the length in characters.
2) Simplified the computation of the start_position.

BUG=512985, 516650

Review URL: https://codereview.chromium.org/1294913006

Cr-Commit-Position: refs/heads/master@{#344342}